As a cop and LE firearms instructor I would be okay with buying a police trade in. Though Id much prefer to see it in person before buying or buy from an online store that has a good condition rating system.
Like others have said, most cops are not gun guys. My department shoots more than most and most of our cops shoot less than a thousand rounds a year. Mandatory training will cover about 500-600 rounds and most patrol cops will come in and practice on their own every couple months or so.
On the other hand you do have gun guys who will take all the free ammo they can get. My gen 3 Glock 22 that I was issued had over 14k rounds through it by the time I gave it back to the department 2 or 3 years later. I shoot my current duty gun, Gen 5 Glock 17, a ton. It's may main practice gun, my competition gun, and my work gun. But it's my personally owned gun so nobody is getting it as a trade in piece.
That's also a consideration. My issued gun, Gen 5 Glock 45, is a back up gun for me. It only has a few hundred rounds through it at this point. It rarely gets shot. It hasn't been out of the safe in months. If I were forced to turn that in and it got traded out, someone would be getting an almost new Glock.