Seems a good idea.

I went the other way 4 years ago and in general for aggressive Offroading it is the norm to go smaller on frame size than you would on a road bike. Couple of reasons- one of which is standover height. You do need more clearance on the top tube on a mountain bike and 4 " is the usual figure quoted. Never had that clearance on any of my MTB's and the Tandem definitely has negative clearance. The other reason is for maneuverability. It is easier to throw the bike around if the bars are closer to you rather than the normal Road stretched out ride position.

But if the bike fits- feels comfortable and you are not going to be doing "Aggressive" offroad- start negotiating the price.