As some others have said the deal is not done until money has changed hands.
I have numerous times had items for sale, people calling to tell me they are on the way never to show up (or heard from again) and as such I never hold an item, unless they are telling me they live some time away. I usually tell them that I will hold it for xx travel time and to call if they will be delayed. In this case I always tell any later callers that someone is on the way over and I would call them back if they don't take it.
Another thing I have learned in the past - someone's definition of in excellent shape may not necessary fit your definition. There are many people out there that don't care about cosmetics (or little things) like some do just so long as it works and they are able to play it. To them it is in good condition since they are able to play it - besides they may not necessary know better either on how the game is actually suppose to look play. Not sure if it is good or bad, but I am one that must have things right or it just won't do (sometimes it costs me some good coin dealing with the small things) - but it is a hobby, not into it to make money.
Of course, I have been on the other side as well with someone offering me a price and show up and then try dickering down more. I personally stick by the original offer and say is it what it is, take it or leave it since I always send them lots of pictures and try to be honest as I can with the true condition. I don't like to hide anything - most of the time it doesn't bother me if they leave as they usually wasted more of their time than mine.
Personally, better to leave if you don't feel right, better than having buyers remorse afterwards and being out of the money and stuck with something you are not going to be happy with. The seller will get over it.