I haven't had many problems on Ebay over the years, but there's certainly a lot of risk if you get a bad buyer. They can open a case months later with Paypal and get their money back, and Paypal will not make them return the item either. This happened to me once, with a RAM kit that the person claimed was an unauthorised transaction several months after the fact. Paypal refunded them without hesitation and with no conditions (like returning the item). Even putting that rare scenario aside, you have various other scams on the table, like somebody just outright claiming that the card wasn't in the box when it arrived and you sent them something else. There's no way for you to prove you didn't (they don't care about photos or even video, as you could have technically still switched things afterwards), so even if Ebay make them return the item, they can just stick a GTX 260 or something in the box, send it back and they'll be refunded. I've never had that one, but I know people who have.
Most transactions go just fine of course, and you have to be really unlucky to meet someone so scummy, but I still tend to avoid Ebay for high value sales these days. I'd rather take slightly less from CeX or the MM on here and have zero worries about getting paid.