Actually I find ebay to be much better than Amazon for buying. I can read the seller reviews and I know they're generally legitimate. Particularly the negative reviews, which is all I need to see if I want to know what the seller does wrong, if anything.
I can look up a seller to make sure he's sourced an item properly, for example, newegg sells SD cards on ebay so I know newegg is sourcing SD cards properly and I'm good to go. Or I look up a seller and they claim to be a licensed Nikon supplier I know I'm probably ok or I could even call up Nikon to verify they're a licensed supplier.
If I want something fake/cloned I know I can just buy cheap from the chinese supplier and I don't have to pay a markup to a middle man. But if I want it right away, I can order from someone in the same state as me and pay a couple extra bucks, and I know it's the clone cuz the picture is the exact same.
You get a lot of information on ebay that you don't get on Amazon.