As with any used purchase, it's been said that...."people respect what you inspect, not what you expect"....
soooooo.....let the buyer beware!!!
With that out in the open, the following is a previous forum thread that covers a lot of ground you need to read b4 buying a VN750:
There's info in the above thread on how to test the charging system, which will of course tell you if the r/r is working ok, as well.
Also: you might stick a small mirror inside the tank (with some help from above with a flashlight) and rotate it around, to see how badly the tank is rusted on the inside. Rusted fuel tanks can obviously cause real fuel system problems later, and corroded fuel tanks are usually never an easy fix.
If you are new to that bike dealer, depending on your own expertise, you might take the bike to another shop of known credibility, and have them inspect it, before you buy.
If the seller hesitates at the above idea, I think you have your answer.
Personally, I wouldn't degoat the bike, unless you plan on going to a completely different exhaust system (good examples in this forum are Vance & Hines, Cobra, and Jardine). Otherwise, the goat's belly gives needed back pressure that provides some of the low-rpm power. You'll miss this power without it.
One last note: these bikes are generally good bikes, but don't buy this or any other VN750 if you plan on loading it up later with lots of electrical accessories. The bike's factory charging system simply won't handle a lot of new goodies. Max stock charging capacity is approx 288 watts.
If you want other opinions on the deal, post the asking price here later, and others will chime in...