Establish a SKU schema for your catalog (e.g., BRAND-CATEGORY-STYLE-SIZE-COLOR) and enforce it through a master product database before pushing to any sales channel
Assign GTINs (UPCs, EANs, or ISBNs) to each unique product variant; acquire GS1-registered GTINs from gs1.org if you manufacture products, or source existing GTINs from supplier data sheets for resale items
Store SKU-to-GTIN mappings in a central lookup table; use this table as the authoritative source when creating listings on eBay, Amazon, Walmart, and Google Shopping, all of which require GTINs
Validate GTIN check digits programmatically before submission using the standard GS1 check digit algorithm to prevent rejection errors from marketplaces
For bundles or kits that do not have a manufacturer GTIN, either apply for a bundle GTIN or use the channel's exemption process (e.g., eBay GTIN exemption request, Amazon bundle exception)
Sync SKU/GTIN data to your shipping software so barcode scanners at the fulfillment center can correctly identify and pick items without manual lookup
Known gotchas
Reusing a GTIN for a different product or a slightly different variant (e.g., different pack size) violates GS1 standards and causes catalog contamination on marketplaces where other sellers share the GTIN
Amazon and Walmart will match your GTIN to their catalog and may override your title, images, and description with the catalog's version; review merged listings for accuracy
Free GTIN sources (recycled barcodes from non-GS1 brokers) are not accepted by Amazon and Walmart; only GTINs with verifiable GS1 Company Prefix origin are accepted for new listings
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp