Although I've had this happen 2 or 3 times, only once was the guy really aggressive about it. I probably would have given him some smart@ss answer just to get him to go away but I didn't want to find out later that my paint had somehow gotten accidentally scratched while I wasn't looking.
Anyhow, next time I'll probably respond with something like; "I don't know and I really don't care because it's not for sale" or "if you really want to know, just go to NADA and look it up for yourself"