Do a little research before you look at the guitar:
Vintage Guitars Info - Gibson collecting vintage gibson guitars
Further look on ebay and other sites to see what price people are currently asking for that model guitar.
I would guess the reason the guy is willing to trade is because he either has no patience for ebay or just wants to get rid of the guitar for some reason. Sound him out and listen to his answers. Could be a fake or stolen guitar. Big guy on Pawn Stars bought a fake gibson mandolin if I remember correctly.