If it's before someone comes to pick it up I don't see the issue, I pulled a sale once on a game but at that point I'd only had a few people offering below my asking price who I had told I'd get back to if i want to go lower.

If you've already driven 2 hours and he knows you are buying it i'd be pissed off.

Everytime I've gone to buy a game I've already agreed on the price and what I usually tell the seller is if the condition matches the description/photos they've provided i'll be taking it. Basically unless you've hidden something about the game it's a done deal. If i then drove out to someone's place and they changed their mind i'd be annoyed, but if I had said I wanted to inspect it and might buy it but not agreed on anything then it's annoying driving there but the seller isn't that bad for changing his mind because up until that point you hadn't agreed to buy it yet especially if you were trying to knock the price down.