Real-time inventory sync
Stock levels accurate everywhere, always. Sell across 30+ markets without ever overselling a single item.
Who it's for
Essential for multi-channel sellers managing stock across platforms
If you sell on more than one marketplace, accurate inventory is the difference between happy customers and account suspensions.
Sellers expanding from one marketplace to many
When you move from selling on a single platform to listing across Amazon, eBay, Allegro, and others, keeping stock accurate becomes exponentially harder. Mastersel automatically synchronizes your inventory the moment you connect your store, so you can add new channels without worrying about overselling. Each new marketplace gets real-time stock data from day one.
High-volume sellers during peak seasons
Black Friday, holiday sales, and flash promotions create inventory nightmares when stock moves fast across multiple channels. Mastersel's real-time inventory locks prevent double-selling even during traffic spikes, protecting your seller ratings and avoiding the cancelled orders that lead to marketplace penalties. Your order management stays clean and accurate through the busiest periods.
Sellers wasting hours on manual CSV updates
If you currently spend 2-5 hours per week exporting and importing CSV files to keep marketplace stock levels aligned, Mastersel eliminates that work entirely. The API-based sync runs in the background 24/7, updating every channel within seconds of any change. You reclaim those hours for growth activities like sourcing new products, optimizing listings, or expanding to new markets.
Inventory Management
Multi-channel inventory management
Mastersel keeps your stock levels synchronized in real time across all channels and marketplaces — letting you sell across global markets automatically, without any manual effort.
Real-time sync
Stock updates propagate across all your channels within seconds of any sale, return, or restock. Every marketplace always shows accurate quantities. The sync covers both parent products and individual variants — so when a medium blue T-shirt sells on Amazon, only that specific variant's stock decrements across eBay, Allegro, and your own store.
No overselling
Inventory locks prevent selling more than you have, even during high-traffic periods or flash sales across multiple channels simultaneously. Critical for scaling operations. The locking mechanism processes stock changes in real time before any competing channel can accept a conflicting order, which protects your seller ratings and prevents the cancelled orders that lead to marketplace account penalties.
Single source of truth
Your store inventory is the master. All marketplace quantities derive from your actual stock — and orders processed via order management keep everything aligned. No conflicting numbers, no data discrepancies. Whether you update stock through your Shopify admin, WooCommerce dashboard, or via bulk import, every change flows outward to all connected channels automatically.
Automatic updates
No CSV uploads, no manual adjustments, no spreadsheets. Inventory flows automatically between your store and every connected marketplace, with shipping quantities always in sync. The API-based integration replaces the 2-5 hours per week that traditional multi-channel sellers spend on manual stock reconciliation, while also eliminating the human errors that cause ghost listings and overselling.
By the numbers
Inventory accuracy at scale
How it works
Always accurate, always automatic
Connect your store once. Inventory stays in sync across every channel without any manual effort.
Store becomes master
When you connect your store, your inventory becomes the single source of truth. All marketplace quantities are derived from your actual stock.
Sales trigger updates
When a product sells on any channel, stock levels update across all connected marketplaces within seconds — preventing overselling. Once shipped, tracking activates automatically.
Restocks propagate
When you add new stock in your store, all marketplace listings update automatically. Monitor everything from your dashboard — no need to manually adjust quantities anywhere.
Why Mastersel
Real-time sync vs. manual inventory
See how automatic inventory sync eliminates the risk and effort of manual stock management.
Without Mastersel
With Mastersel
FAQ
Inventory sync FAQ
How fast does inventory sync?
Inventory syncs in real time — within seconds for both sales events and stock changes. For example, when a product sells on Amazon Germany, stock is automatically decremented across Amazon France, eBay, Allegro, and your own store — all within seconds. This covers both decrements from sales and increments from restocks or customer returns. Every connected channel always reflects your true available quantity without any delay.
Can overselling happen?
Overselling causes cancelled orders, negative customer reviews, and marketplace penalties that can suspend your account. Mastersel's inventory locks work across all connected channels simultaneously — when an item sells on one platform, it is instantly reserved everywhere else. Returns automatically restock inventory too. Even during Black Friday traffic spikes or flash sales, the system prevents double-selling by processing stock changes in real time before any other channel can accept a conflicting order.
Where does inventory data come from?
Your Shopify or WooCommerce store serves as the master source of truth. When you update stock in your store admin — whether adjusting a single product or bulk-importing new inventory — all marketplace listings update automatically. Mastersel supports variant-level sync for attributes like sizes and colors, and bundle or kit stock is calculated from component inventory. This ensures every channel reflects accurate availability down to the individual variant.
Do I need to upload CSV files?
No. Traditional multi-channel sellers spend 2 to 5 hours per week on CSV exports and imports, and errors in CSV files frequently cause overselling or ghost listings that frustrate customers. Mastersel's API-based sync eliminates this entirely. There are no scheduled batch updates — everything flows in real time between your store and every connected marketplace. Stock changes propagate automatically the moment they happen, with no automated processing delays.
Are there per-SKU fees?
No. Inventory tools like Linnworks charge €100-300/mo, and ChannelAdvisor starts at €500+/mo. Mastersel includes unlimited SKU sync completely free. Even sellers managing 10,000+ SKUs pay nothing for inventory management. There are no per-SKU charges, no sync fees, and no usage limits — track stock levels in analytics and manage inventory at no additional cost.
How does Mastersel inventory sync handle products with multiple variants like size and color combinations?
Mastersel syncs inventory at the individual variant level, not just the parent product level. Each size-color combination — for example, a blue T-shirt in size M — is tracked as its own stock unit across every connected marketplace. When that specific variant sells on one channel, only that variant's quantity decrements everywhere else. This prevents the common problem where selling one variant incorrectly reduces stock for a different variant. Bundle and kit products are also supported, with component-level inventory calculations ensuring that selling a bundle correctly reduces the stock of each included item across all channels.
What happens to my inventory levels on marketplaces if my Shopify or WooCommerce store goes offline temporarily?
Mastersel maintains a cached copy of your latest inventory state, so a brief store outage does not cause marketplace listings to display incorrect stock levels. During the offline period, any marketplace sales are still processed and queued. Once your store reconnects, all pending inventory changes are reconciled automatically — stock decrements from marketplace sales are applied to your store, and any updates you made before going offline are pushed to marketplaces. This ensures data integrity is maintained even during unexpected downtime, without requiring any manual intervention on your part or disrupting your shipping workflow.
Why real-time inventory synchronization is critical for multichannel sellers
Overselling is the single biggest operational risk in multichannel e-commerce. When a product sells on one marketplace but stock levels do not update on other channels, subsequent orders arrive for items that no longer exist. This leads to order cancellations, negative reviews, marketplace penalties, and — in severe cases — account suspensions. For sellers operating across 30+ European marketplaces, the window between a sale and a potential oversell can be seconds.
Mastersel eliminates this risk with real-time inventory synchronization that operates at the variant level. When a product sells on any channel — your own store, a European marketplace, or any connected platform — stock levels update across every other channel within seconds. When you receive new inventory and update stock in your store admin, every marketplace reflects the change immediately. This removes the need for manual stock adjustments, safety stock buffers, or marketplace-specific inventory reserves that artificially limit your sales potential.
How stock discrepancies destroy marketplace performance
European marketplaces track seller metrics closely. Order cancellation rates, shipping delays, and stock accuracy all factor into your seller ranking and search visibility. A single oversell incident can trigger a warning; repeated incidents can reduce your listing visibility or suspend your account entirely. For sellers using manual inventory management (spreadsheets, CSV exports), maintaining accuracy across multiple channels becomes impossible at scale — especially during high-volume periods like Black Friday when orders arrive every few seconds. Automated sync is not a convenience feature; it is an operational necessity for serious cross-border sellers.
Variant-level accuracy across every channel
Inventory sync at the product level is insufficient for sellers with variants. A t-shirt listing might have 20 variants (5 sizes x 4 colors), and each variant has its own stock count. Mastersel syncs at this granular level, ensuring that when a Medium Blue sells out on one marketplace, it is marked as unavailable across all channels — while Large Blue remains available everywhere. This precision prevents the most common type of overselling, where a product appears in stock because other variants exist, but the specific variant ordered is already depleted. Combined with multichannel selling and unified order management, variant-level sync gives you confidence to list your full catalog without fear of stock errors.
Multi-channel inventory sync at no extra cost
No sync fees, no per-SKU charges. Shipping included — completely free.