Personally I do not like the way your LBS sounds, and I would look else where, but if you are stuck on that bike try this...
If he has two other sets of the same model of disc brakes, then could you have him take off the front set and put on one of the other front sets, that way if it is the rotor, or something else on that set, you would not be stuck with it
Personally if he shoots down that idea, then you know there is something wrong with the set that he is tring to sell you
I would watch him personally and try the bike out before you buy it