Amazon holds or guarantees inventory. If you buy from a 3rd party and get ripped off, you get your money back for sure. If something shows up broken, Amazon replaces it. If something is bothering you about your transaction, you can call them. And most things show up on your door in two days. Plus, Amazon knows what I want to buy and forces me to do it!
Ebay lets any idiot who says he has something to sell, sell it. There's plenty of buyer- and seller-side fraud, but Ebay only really punishes sellers. As a practical matter, you have to use Paypal, which acts like a bank but isn't regulated like one. The UI isn't really meant for casual shopping.
Amazon is where you go for newer stuff, Ebay is where you look for things that you might otherwise want from a yard sale. Both have their place.