Umm i dont know exactly.
There could be scams... My friend bought some shoes off eBay, and they were worse than the real ones. They were terribly made and literally fell apart within weeks of use.
On the other side, I have bought many things off eBay, and have yet to be upset.
Im guessing that a lot of people will bid on that item at the very end of the alotted time, (which is what people do on ebay... bid right as the item runs out of time), or there might be a reserve price. The reserve price is a price that has to be exceeded for the item to be bidded off.
So if the reserve is $100, and you bid $20, you wont get it because it didnt get past $100.