You may want to Google the M&P 9 and extraction problems. They are fantastic pistols. But, there does appear to be a not-uncommon issue with feeding/extraction reliability.
S&W has been very good about taking (thousands?) of them back for service, which I've been told mostly involves a new extractor and polishing the chamber more completely. Some of them, including a couple I know of personally, have not been fixed satisfactorily, even after a couple of trips back to the mothership.
Many of the M&Ps that have had issues are now out on the used market as their former owners gave up. This is definitely true of the several I know of which S&W did not get running correctly after one or more repair shop trips.
So, for my money, while I still think the M&Ps are a wonderful, very ergonomic design with a lot going for it, I don't think I'd buy a used one. I'd save the cash and buy new, hoping to better my odds of getting one that works right out of the box, or that just needs one trip back to S&W for fixing.
With any used gun, you run the risk of buying someone else's chronic problem gun that they've given up on. With the M&Ps, the used market has the potential to be a minefield of these lemons.
Of course, maybe the former owner just wanted a change...after only a few boxes of ammo. Only one way to know...
-Sam