I don't get ebay. Everyone seems to think that it's the best thing ever, but it's weirdly organized, and so haphazard with these auctions that go on for a few days. So if you manage to catch something on there that you think you want, then you don't know if you'll get it or not. And, chances are, you'll what it is you're looking for.
I've never bought anything from ebay. Too confusing.
On the other hand, I like Amazon's marketplace. There's a whole bunch of rare and obscure stuff on there, the price is given to you up front, and it's easy to search. Last week, I bought a book about how to buy a car for $4. Tonight, I bought a comicbook about a photo developer for $8. I feel quite satisfied with the whole arrangement. I understand the principle. It works -- things tend to show up within a week. Perhaps the only thing to complain about is that the postage is at least $3.50, meaning that you can't really buy anything super cheap -- but that's hardly a great complaint.
Final thought: it still took me 1/2 an hour to find the car buying book that I wanted at a price that I liked. If it takes me this long to spend $4 on a book, how long will it take me to actually buy a car?