Why SKU consistency is the foundation of multi-channel sync
Every inventory sync problem ultimately comes back to one thing: can the system tell that product A on Platform 1 is the same physical item as product B on Platform 2?
If the answer is yes, the system can keep their inventory counts in sync automatically. If the answer is no, everything has to be done manually.
The way systems make that determination is by matching SKUs. When your Etsy listing for "Handmade Silver Ring Size 7" has the SKU RING-SLV-7 and your Shopify product for the same ring also has the SKU RING-SLV-7, any inventory sync tool can match them with certainty. When the Etsy SKU is HSR07 and the Shopify SKU is silver-ring-size-7, nothing matches automatically. every link has to be created manually.
For sellers with 20 products, manual matching is inconvenient but manageable. For sellers with 500 SKUs across 4 platforms, inconsistent SKUs make real-time inventory sync nearly impossible to set up reliably.
The different product identifiers: SKU, ASIN, UPC, EAN, barcode
Before building a SKU strategy, it's worth understanding how each type of product identifier works and when you need each one.
SKU (Stock Keeping Unit)
A SKU is a code you assign yourself. It's internal to your business. It uniquely identifies a specific product variant in your inventory system. SKUs don't need to be registered with anyone. you make them up. This is the identifier you control completely, which is why building a consistent SKU system is so important.
UPC (Universal Product Code)
A UPC is a 12-digit standardized barcode number issued by GS1. UPCs are globally unique. no two different products share a UPC. Amazon requires a UPC or GTIN for most new listings. Walmart Marketplace requires them. If you're manufacturing or sourcing branded products, your items may already have UPCs from the manufacturer.
EAN (European Article Number)
The EAN is a 13-digit barcode standard used primarily in Europe (Amazon calls it a GTIN in its generic sense). For multi-national selling, you'll encounter EANs on imported products. For practical purposes, treat EAN and UPC as the same type of identifier. both are GS1-issued global product codes.
ASIN (Amazon Standard Identification Number)
Amazon assigns an ASIN to every product in its catalog. ASINs are Amazon-specific. The same physical product might have different ASINs for different Amazon marketplaces (US, UK, Canada). You can't control ASINs. Amazon creates them. What you can do is ensure your SKUs map to the correct ASINs so you're always listing against the right product page.
How they relate
Think of it this way: the UPC identifies the physical product universally. The ASIN identifies the Amazon catalog entry. The SKU is your internal tracking code. A single physical product might have one UPC, multiple ASINs (per Amazon marketplace), and the SKU you chose. Your inventory sync system needs to know which SKU maps to which UPC and which ASIN to function correctly.
Building a SKU naming convention that scales
A good SKU naming convention is:
- Consistent. same format for every SKU
- Self-describing. you can identify the product by reading the SKU
- Compact. under 20 characters, no spaces or special characters except hyphens
- Scalable. the format works for your 10th product and your 1,000th
Recommended format
The most practical format for multi-category sellers is:
CATEGORY-PRODUCT-VARIANT
Examples:
- RING-SLV-7. Ring, Silver, Size 7
- SHIRT-BLU-M. Shirt, Blue, Medium
- MUG-WHT-12OZ. Mug, White, 12oz
- NECKLACE-GLD-18IN. Necklace, Gold, 18 inch
Rules for your SKU format
- Use hyphens as separators, not underscores or spaces
- Use uppercase letters for readability in spreadsheets
- Use abbreviations consistently. if "BLU" means blue in one SKU, it means blue in all SKUs
- Never reuse a SKU, even after discontinuing a product
- Don't include price, year, or date in the SKU. those change
- Use the same SKU format across every platform, always
Write out your full SKU format in a document before you create any SKUs. Include your abbreviation standards for every category, color, size, and material you use. Share this with anyone who helps manage your inventory. Consistency across every person who creates SKUs is what makes the system work.
How each platform handles SKUs differently
Understanding each platform's relationship with SKUs lets you build your catalog in a way that minimizes friction.
Amazon
Amazon calls your SKU a "Seller SKU" and it lives at the offer level (your specific listing), not the product level (the shared ASIN). Amazon's systems use the ASIN as the primary product identifier. Your Seller SKU is how you track which ASIN is yours. You can set your Seller SKU to match your internal SKU. Amazon doesn't dictate the format.
For multi-marketplace selling (US + UK + CA), note that the same Seller SKU on different Amazon marketplace accounts refers to different listings. Keep your Seller SKUs consistent across marketplaces to make cross-marketplace inventory management workable.
Etsy
Etsy has a "SKU" field in each listing and each variation. This field is optional on Etsy but critical for inventory sync. Fill it in for every listing and every variation using your standard SKU format. Without a SKU, Etsy listings can only be matched by title. which is unreliable.
eBay
eBay supports a "Custom Label" field on each listing. this is functionally a SKU. eBay also has SKU fields at the variation level. Use your standard SKU format in the Custom Label field. eBay's inventory management system can use these Custom Labels for tracking and bulk operations.
Shopify
Shopify has a dedicated SKU field for every product variant. It's prominently positioned in the product editor. Shopify's inventory system uses this SKU field for its own tracking and for third-party app integrations. Shopify is typically the cleanest platform for SKU management.
Mapping existing SKUs when they already differ
If you've been selling on multiple platforms for a while, you almost certainly have inconsistent SKUs. Different formats, different abbreviations, or no SKUs at all on some platforms. Here's how to clean this up without starting from scratch.
Export your catalog from every platform
Most platforms let you export a CSV of all your listings with their current SKUs (or Custom Labels). Export from each platform and put them side by side in a master spreadsheet.
Create a mapping table
Add columns to your master spreadsheet: Platform 1 SKU, Platform 2 SKU, Platform 3 SKU, New Standardized SKU. Match each product across platforms by hand. This is tedious once but permanent.
Update SKUs on each platform
Use each platform's bulk edit tools to update the SKU field to your new standardized format. For Etsy and Shopify, you can do this via CSV import. For Amazon, use a flat file update. For eBay, use File Exchange or the Bulk Edit tool.
Connect your inventory sync tool
Once SKUs are consistent, Commerce Kitty can auto-match your products across platforms. The matching process that previously required manual linking now happens automatically based on your consistent SKUs. For the full picture of how SKU matching fits into your day-to-day workflow, see our listing management guide.
SKU strategy for product variations
Variations are where SKU strategy gets most complex and most important. See our complete guide on managing product variations across platforms for the full treatment. Here's the SKU-specific summary:
Every variation. every unique combination of attributes that has a separate inventory count. needs its own SKU. A shirt with 4 sizes and 3 colors has 12 variations and needs 12 SKUs:
- SHIRT-BLU-XS, SHIRT-BLU-S, SHIRT-BLU-M, SHIRT-BLU-L
- SHIRT-RED-XS, SHIRT-RED-S, SHIRT-RED-M, SHIRT-RED-L
- SHIRT-GRN-XS, SHIRT-GRN-S, SHIRT-GRN-M, SHIRT-GRN-L
The pattern is clear and consistent. Anyone looking at the SKU knows exactly which item it refers to. Any system can match SHIRT-BLU-M on Shopify to SHIRT-BLU-M on Etsy without ambiguity.
The mistake sellers make is using a parent SKU (SHIRT-BLU) and then adding size as an attribute separately. Some platforms handle this fine, but it makes cross-platform inventory matching at the variation level much harder.
For the next step, see how to track inventory across multiple platforms once your SKUs are in order, and our guide on managing product variations across platforms for handling complex catalogs.