What if (and I hate what ifs):
Someone agrees to buy your item. Tells you they are sending the money... a few days go by and they never paid. You send them PM's asking about it.. nothing. You sell to someone else. Then all of a sudden you receive payment from the person.
I guess what is the "reasonable" timeline between committing to pay and actually paying.
I don't think letting the buyer and seller figure this out on their own will suffice as 2 people could have very different opinions on what is except-able. Maybe there could be a set rule, that if followed would protect the seller from negative feedback.