im 17 now and last summer i made over $1000 buying/selling/trading. it sure helped pay for gas and taking my girl out. this is a pretty common practice at gun shows.us i would say 25% of the people that come are there to make money.
It's possible the seller didn't buy the lowers from AIM surplus, they bought them from somewhere else (like maybe joeboboutfitters.com), had to pay a transfer fee and didn't make as much profit as you would think