This is a fairly basic scam which I have had attempted against me on several occasions.
The auction proceeds as normal. The buyer (frequently a low or zero feedback bidder) wins the item. On receipt of the item, the buyer then contact the seller and claims that the item is not as described (faulty, the wrong size, etc), and asks to arrange a refund, including p+p costs for the return of the item. The seller agrees and refunds the sale price, the original p+p and the return p+p. The buyer now has the item for free, plus a small profit from the return p+p. All the buyer has to do is claim to have dispatched the item, and the seller has no recourse.
Sellers are vulnerable to this mainly because of the threat of negative feedback. Here are some ways around it:
1. Be sure that the item you're selling is exactly as described in the auction. If you make absolutely sure of this then the buyer cannot claim otherwise.
2. Don't leave feedback for the buyer until you're sure they've received your item and will not ask for a refund. If you leave them feedback as soon as they pay, you will only be able to amend the comments, but not alter their score.
3. Tell the buyer you will only give them a refund once you have received the returned item and found their claim to be accurate. A scammer will imediately balk at this and will usually fall silent.
4. Don't deal with zero feedback eBayers. This one is not ideal, but unfortunately as it is necessary for people to protect themselves from fraud (buyers and sellers alike), it may be necessary to simply ask that zero feedback bidders do not participate in your auctions, or that they at least contact you before bidding.
If the scam attempt fails, the scammer may choose to leave you negative feedback. If they do so it is vital that you raise a dispute with eBay as soon as possible to get it removed. This should not be a problem if you have generally good feedback and the buyer has low or no feedback. In all cases, it is imperative that if possible you leave details of the exchange on the buyer's feedback comments, to warn other eBay sellers who might encounter the scam.