From what I understand, as long as they haven't already accepted, no deal was struck. You must formally rescind your acceptance (I'd save a copy of the IM as well for CYA purposes).
I had a similar thing happen. Someone asked if I'd take $XXX, I accepted, and they became a ghost. A week or two later, someone else wanted it. I still went back in my IM's and sent a message to the ghoster, rescinding my acceptance. The guy was very confused