You can pick up a good quality Cybergun/King Arms Thompson for $200 or so, I believe? Brand new, no worries about quality or previous mileage.
We can talk Thursday or whenever we meet up.
Without knowing this fellow, it's hard to say how he has treated the weapon and what he could have done with it.
(WD40 down the barrel, etc..) There are a fair few things he could have done to it, that one should never do.