When you have something for sale and a person is asking questions about it's condition before buying, and someone P.M.'s them saying "I'll take it" why not tell them that you are discussing it with someone, and I can tell you withing the next 5 mins if is still available. Instead of selling it out from under them?
If they are a serious buyer, they still will be in a few minutes.
Better yet why not prepare your ad and give a complete description and photos in the first place?