I have bought lots of things on ebay & most of the time it's fine. have had a few problems such as
1) a seller who hadn't put a reserve but decided after the auction had ended that he hadn't got a high enough price so refused to sell the item
2} someone who had a second person who did a series of shill bids up to my max bid then withdrew the one that beat mine simply to raise the bid price.
3) a guy who not having got as high a price as he'd probably hoped for & who's listing insisted on having the item paid for by paypal before posting, - then claimed it had been stolen and sent back a check which he conveniently forgot to sign.
4} a couple of sellers who stated collection only & then made it impossible to collect: one by initially not answering messages and then not supplying an address, another by not being at the collection address & not answering her mobile when I'd travelled an hour & half towing a trailer to collect some white goods.

The one with the unsigned check is still a matter being resolved. The one that annoyed me the most was the waste of my time & fuel in a fruitless journey. Sometimes the problems are misunderstandings and can be sorted out , sometimes the only recourse is to report the person to ebay & there have been a number of sellers where shortly after I reported them there was a message stating that this person is no longer a member.

There can be problems with using ebay mostly to do with either mismatched expectations or because it's buying at a distance but the majority of ebayers are genuine.