i suppose it depends on the leniancy of the seller, or the circumstances of a fallen through sale
on ebay, as rules state, people bid to buy, and if i listed something on ebay i'd list it as full and as accurate as possible and if someone bids but then decides not to buy then i would report them to ebay, meaning i may be able to get seller success fees refunded, however there are a small number of people who bid with a genuine interest but unforseeable circumstances mean they cant complete the sale, but then it's up to you what you do about it if its your item that sold but didnt sell!
feedback is a very important thing, including the justifiable negatives innocent people get from idiots who leave negatives for petty reasons!
not sure if i mentioned this on a related thread but I once bought an o2 XDA off ebay for £120 off a guy who appeared to be genuine, I paid with paypal, only to get an e-mail from the seller telling me that the account my money had gone to wasnt his, and that his ebay account had been hacked, I immediately complained to ebay and to paypal, paypal froze the payment and attempted to contact the paypal-registered e-mail, with no luck, so the transaction was reversed, but also ebay refunded me the money I'd paid, minus £15 admin fees, not realising that paypal had managed to stop my money leaving my account in the first place, so I got my £120 back through paypal and an extra £105 from ebay

I left £120 in my paypal account for a month to give them chance to rectify their mistake, but they didnt... quids in!!