Asking price isn’t getting price. I recently went looking for a second U22 Neos and came away shaking my head. Granted, mine was purchased not long after their introduction but I’m hard pressed believing they’ve doubled in price as a starting bid on used. Same story with tracking down a carbine kit which turned up 1 result at $1,200. Great expectations.
The shell casing bit is correct, my M&P came with one to send to MD if I happened to live there. Now as far as unique marks go, plenty of people dress the extractor and ejector on a new pistol, the firing pin can be modified or cheaply replaced, and chamber can be polished which alters the semi-unique markings left on spent brass.
Now IF the upper were used in a crime it would still be traceable only as far as the original owner unless he kept records.