The eBay ranking system relies on the users to post feedback on their experience with either the buyer or seller, depending on their perspective in the transaction. So each eBay user gets a total feedback number, as well as % indicating positive, negative or neutral. The higher the feedback rating with the higher % positive, the greater your likelihood of having a positive transaction experience with that buyer or seller.
For example, I would NOT purchase a big-ticket item from anyone unless they had feedback rating of over 100 (as a seller -- keep in mind whether you buy or sell, your rating score totals the same. You have to actually go in and look at the individual transactions to see if that user is predominantly a seller or buyer, or a mixture of both), with nothing less than a 95% positive in ratings. You can also go in and see why someone gave a user a negative rating. Sometimes this is helpful, because you never know what the circumstances are. Sometimes things don't work out on a transaction for legitimate reasons.
Hope that helps. I would try to spend some time on eBay to familiarize myself with the rating system so that when you DO find something you're interested in you'll know how to assess the seller.
I've bought everything from fine art and furniture to teacups and jewelry on eBay. I've also sold stuff. Only had one really bad experience from someone who fraudulently represented something (and it's my fault for not paying attention to their feedback score, which was sketchy at best).
Good luck! And remember, buyer beware!