Apples and oranges.
If you're buying a one-of-a-kind item for example that you would buy at an auction house, then ebay. I miss the old ebay where it was mainly focused around auctions, not just buy it now items. You used to be able to go there and find unique items. Granted, this is still possible, but ebay has certainly geared more towards commercially-sold products.
Amazon is more like a virtual brick and mortar store that sells lines of particular products rather and singular, unique items.