I would say, just don't use him to buy his guns cheap and then flip them. That would p*ss me off if someone did that to me. He most likely knows the market and was just giving you a deal.
If I gave a buddy a deal, knowing full well what I could get out of it, then they bought it and sold it for a profit it would make me mad. I rarely give "buddy" deals anymore, sometimes, but not often.
I have experienced this far too much. No more.