When I've sold guns via the internet, I've always had an unspoken rule that if someone opens a line of communications on a gun with me, by emailing or sending a PM, then they in effect are like the customer in a gun shop who is looking at a gun. The next guy can have it when the first one passes on it. It has happened that way a couple of times, and I think it is the only fair way to go about it.
If I owned a gun shop, I would never pull a gun from a customer's hands ( I have had that done to me btw), who was looking at it, just because the next guy to come along says he wants it. The internet shouldn't be any different.