Seriously - what does a buyer expect????
The short answer is...honesty!
Agree 100% . I've only ever sold a couple of pins (all outside of the forums) and one thing I do is give as much information as possible AND always state to a potential buyer that if there's anything else they want to know then ask away . This ensures that there are no hidden "surprises" when the buyer gets the pin as everything should be up front before the sale takes place .
You sometimes get people who don't ask a lot of questions however so the best thing to do as a seller is to be honest and upfront to cover yourself if the buyer is unhappy later . (NOT saying that anyone here is dis-honest . It's just what I do myself)
I think the idea of a feedback system etc. is a great idea or perhaps keep a log of who has sold pins to who etc. that way members can privately contact previous purchasers from the same seller via PM to get their opinions.