and dont forget a lot of people are just unknowledgable/dont know any better. He may have bought it from some other guy who told him it was a 240, and that guy bought it from another guy who told him it was a 240. And who knows, at one time someone (even the guy selling it now) might have had it bored to 67mm, which is .040" over or 1mm over stock, but people are really so confused that they will think "well, its 200cc, and the machine shop guy said now its "Forty Over" so now it must be a 240cc", when its actually 203cc or whatever, you would not believe how much stupid things like this happen.
Pretty much the safest way to buy anything with a motor is to know as much as you can yourself, then ask the seller questions about everything and see how much he knows about bikes, if he doesnt know much, you have to just look and learn for yourself because chances are if he doesnt know squat, the info he gives will be wrong. If you dont really know all that much about the bike/motors, and neither does he, then good luck, because you're taking a crap shot.
The bottom line is really the more you know, the better off you will be, and if there is no way to tell that it is a 240, and he does not have any proof that it is a 240, then he either needs to remove the head and measure it for you, or he cannot sell it as a 240, and offer him the value for a non-240 similar blaster. I would definitely ask him if he did the 240 or was he just told by the last owner it had a 240. If he was told by the last owner, I'd be careful, because he could have been duped, not to mention that would mean that he never rebuilt it since he bought it and it probably needs work.
You also have to remember that this guy may think he is telling you the truth, he may honestly believe its a 240, either because he was told it was, or he is confused, or maybe it really is a 240; the problem is that you can never really tell without looking yourself if there are no obvious ways to tell.
