There are multiple M&P's out there, so some clarification may help your cause.
I always think of a new gun as more valuable than a used gun when it comes to guns currently in production. Thus, a shield will probably not sell for more than 350, a full size in the 350-400 range, and the ar styles will sell in the 650 range bare. Of course any add-ons could increase (or perhaps decrease) price but that depends on buyer too.
Hard for a buyer to buy used for near the new price of a readily available weapon