I once traded a Nintendo 3DS for a laptop with an i5, 8GB RAM and an SSD. If a deal is a deal, do it.
Also, #1 rule of this is to block numbers/contact after. He'll probably realize at some point hes been conned, so best action in my books is to never let him contact you for money or to get the system back.
Would be an idea to post the link or a screenshot of the AD so we can check it over, no harm in having a 2nd/12th pair of eyes on it for anything dodgy.