jwolthuis - 3:33 pm on Jul 4, 2012 (gmt 0)

Amazon does require UPCs (barcodes) but somehow I've seen some products listed by my competitors that I know don't have UPCs.

Not true. I sell products on Amazon, stocked in Amazon's warehouse (or not), where I've applied an ASIN sticker, prior to shipping it to the warehouse. No UPC required. In my genre, few products have any form of GTIN.