Just my personal opinion, but I'd rather buy from a dealer considering I have to wait that amount of time for the stamp to come back. If someone is going to hold my cash, it'll be either someone I know personally or a dealer who I have legal recourse more easily should things go south.
Things like this needs a type of BOS. Especially if it was being dealt in large sum of cash. Paper agreement can be written too where buyer can pay in full when stamp comes back approved. I think that would be better option since the approved stamp goes back to seller and if buyer doesn't pay, seller can cancel the tax fee.

