Quirrel – you mean if you win something and the seller won't give it to you? The solution is to buy from people with a good feedback record (for selling as well as buying) – if you don't get the goods, you tell eBay and they can enforce or get you your money back, but any hassle and you say so in the feedback. The seller isn't going to risk that by messing you about.
But you need a seller with selling feedback – it's been known for people to buy a number of small items, get 100% feedback by paying promptly, then sell an expensive bike and disappear with the money. They don't care about back feedback because they're never going to use that account again. Which is one reason why I like to pick up bikes in person and pay cash.