Mark,
Selling equipment "as-is" often provides an implied warranty of merchantability, a promise that the equipment can be used as expected given its type. Most states will allow businesses to disclaim implied warranties, including a written disclaimer with the sale. The customer should sign the form to acknowledge the disclaimer. The bill of sale needs to be specifically worded to include aftermarket installed items with the possible safety caveats.
The problem is it will cost you five figures to find out whether you have recourse or not. I understand what you are stating and our seller seems to be sincere in the rifle's sale, and he more than likely knows all the pit-falls involved in selling altered or modified firearms.