eBay does not use a consolidated product listing, much more like a bulletin board each sellers products being sold under it's own URL. Amazon consolidates the listing for all sellers into a single product and uses their own ranking algorithms to determine which seller is in the "buy box" (which accounts for 97% of the items sales velocity). Amazon treats the entire experience as their transaction, they anonymize the emails and don't allow sellers to send follow-messages to clients or market other products to them (package inserts). The cart-checkout process isn't broken down by seller either, it's a unified payment and "1 click" checkout. I don't feel bad for Amazon at all - they've had it coming!
FWIW .. I was the first person (as a 3rd party) to develop an integration with Amazon's marketplace. I also personally won eBays' star developer award. I recall the year the Internet Retailer show was in San Diego and Amazon didn't have an employee there. It was against Amazon corporate rules to allow employees to spend the night in CA in fear it would create sales tax nexus. Amazon employees would need to fly back to Seattle and then fly back to San Diego if they came down for a friday meeting and wanted to spend the weekend on personal time! Amazon has gamed the rules, they compete directly with their clients (sellers) using the sales data to decide what product to source. They ran their warehouses as 3rd party entities to avoid nexus and begrudging accepted 3rd party (non-Amazon owned) sellers when the courts ruled that wholly owned entities would create sales-tax nexus. I feel no pity for them.