I am pretty sure its due to people trying to finance AR purchases.
That's how I just got a used Savage 111 in 270 with scope sling, detach box mag and 48 rounds for $290!! (Although for him it was an AK he was after.) First time shooting it was a mixed bag, I actually have a thread about it right now. I know not all used guns are good guns, but I'm very happy with my purchase.