xenalook - I suppose it depends. In my selling area, used computer equipment, for every 100 items I sell, I have to file for about 7 FVF credits. A few people ask me nicely to back out, and I let them. However if people go "oh, I didn't realize this.. please help" and -hint- at what they want me to do without actually asking me directly, I can't help them. Perhaps I'm jaded after years of selling, but I have plenty of people who come up with all sorts of colorful excuses to back out when if they'd actually be up-front with me, I'd let them. A lot of scammers use such tactics as well.
Buyers want sellers to be up-front in their descriptions, selling terms and ship costs, and conversely I want buyers to be up-front with their concerns or to admit their mistakes instead of giving me a nudge nudge wink wink treatment.