Have you checked cycletrader? If not, do a nationwide search to get some parameters on what your bike is listing for as an asking price and what his bike is listing for as an asking price. Then, if the numbers look reasonable, you need to see his bike (if that is impossible, asked for pictures from every angle and copies of service records). If it still looks good, I would consider the tire issue silly, but I would be willing to negotiate that number down a bit (unless he just put them on, as in less than 100 miles). The only 954 I personally know about is a guy I see at trackdays with one. He has beaten on it for a few years and it has never broken.
Good Luck, Steve