I wouldn't have a issue with someone paying for me to fly a fun cross country in my plane, I also think this makes the buyer have some real skin in the game and is more likely to purchase your plane.
That said, if you can't be there for it, I wouldn't do it.
Also if this is just a prebuy and not a annual or 100hr inspection I would scan all the logs and paperwork and only give access to those copies, I wouldn't allow his AP to so much as touch the actual logs, let alone write in them, write up that contract that you affirm that the copies are full and accurate, and if the sale goes through you will provide the actual logs (bring them with you but leave them in the hotel), also write up that at the end of the prebuy the plane is to be in the same condition as when it entered the prebuy (i.e. not disassembled if the mechanics feels he found something).
It amazes me how many people buy and sell planes without being physically there, must have a lot more money and faith than I do, also I wonder how these people even find time to fly their own planes of they are that busy.