In my experience, used guns seem to sell reasonably quick when prices around 25% below current retail pricing. Accessories can go a bit to either side of that number.... Guns that are priced higher seem to sit for at least a few weeks.
So, if I were looking to buy a .40 S&W M&P, I would look around for the lowest price I can find a new one from a dealer (which is currently $649 + GST = $688) and subtract about 20% - 25% depending on the condition of the gun. So (at least for me) the most I would pay for a used M&P is around $550, and maybe $30 - $35 each for the used mags. So maybe $690 plus shipping for the package, if it is all in really good condition.... Anything higher than that and I might as well buy it all brand new from a dealer. But that's just me. YMMV, of course.