This is a good idea.
The seller can check "on hold" or "pending payment" or some such. This doesn't expire the ad like "sold" does. It would still be on display, but the "contact" seller wouldn't work. Instead if someone clicked "contact seller" they'd get a message like: "The seller has put the item on hold at the present time. Check back later to see if this hold is released or if the item is sold."