Outside of ebay (craigslist, for example), there are man-in-the-middle scams. The scammer agrees to buy bitcoins from you and to sell something to someone else. They give the other person your paypal account. Then the other person sends you the money and you send the bitcoins to the scammer. When the other person doesn't receive the item the bought, they get contact PayPal and get their money back (note they were actually scammed in this case). The scammer keeps your bitcoins.