don't try to screw him; he seems like a guy who just made an honest mistake... i'm sure he'd love for you to just pay him $10.00 extra and you keep the D-Link, but if you check the D-Link auction, someone else won the modem (that you now have), and in the end that person's going to be the one that suffers.
if he was going to "scalp" you, he'd have done it already... the fact he sent you the modem in the first place is proof that he's an honest person. it's a mistake that could happen to anyone, and that i've experienced a few times myself.
i say send it back to him, and let him send you the modem that you rightly won