According to EAN/UPC barcodes rules, only one can be listed for any one product. Therefore Multi-vendor sellers are unable to list a product another vendor has it with the barcode, as products are required with bar-codes to sync with any other systems, such as Google Adwords, Amazon, etc.
The issue is the vendors can have the same product lines, so it would be a case that lets say they could list some items and then not others depending on what was added already including the unique EAN. The other workaround is that vendors list the items and dont have EAN bar-codes which means they cant be synced with systems that require it, this also causes an issue because you could have 10 of the same items listed. How does the customer know which one to purchase?
The solution is that Multi-vendors can sell their product via the same EAN on each product page, listing the cheaper option first to promote the lowest price. This is how Amazon works, it seems like the only feature missing from X-carts multi-vendor experience.