It may be difficult for admins/mods to find proof of what either side may be claiming. If it's written on a thread (offer to buy by a buyer or commitment to sell from a seller) and the other party never follows through, then that may be enough. I don't know if the admins can even go over any private convos on DNF (they might if the user consents but ideally PMs for both profiles would need to be looked at), or something may be said by email/social media outside DNF
If a good long-standing member gets screwed and the other guy has questionable reputation or is really new then they may act given the circumstances. Promises can be broken on both sides, sellers may try to back out if they get greedy or buyers may stall/not pay not allowing others to make offers while sale is pending