I once heard some very wise advice about this exact situation.

Any time a buyer gets their asking price they will be happy. If an old lady lists an Addams Family for $350 and it sells for that price, she will be happy. Yeah, she'd be even happier (and surprised) with 5 grand, but I digress. What you don't know can't kill you. Personally in a situation like that, if the deal was TOO good, I would throw a few more hundred dollars on top if I was wealthy after it is loaded in your car or whatever. If the deal was only amazingly underpriced, I would just jump on it myself, but no, I wouldn't meddle in the business of letting the seller know or offering more if I knew other people were coming. If I didn't know, maybe, if he could hold. Then you're just paying him more for the service of holding it anyways.