There are a couple of regular scams I've come across (not happened to me but to others):
One bloke auctioning a rare record. It goes for less than he wants, so he refunds the seller with a message along the lines of "I couldn't actually find it in my collection/it's scratched/a mate has borrowed it, sorry."
A couple of days later, a new ad from the seller for the same item.
Another one was for car seats, this is worse. Ad says collect in person, bloke pays in advance by PayPal, collects the seats in person, all smiles, then opens up a PayPal dispute saying he never received the seats in the post. As seller has no proof of post, PayPal paid the money back to the buyer.
Even though the seller had written receipt, PayPal said he could have forged it, so wouldn't have any of it. Massive flaw, beware.
So now, I stipulate cash on collection on anything they are picking up locally. If they try to pay me with PayPal, I'll immediately refund it until they turn up with the cash.