Police trade-ins come from two major trade-in systems.
First is when a department is changing weapons.
Often a department is dissatisfied with a particular brand, model, or caliber gun.
After the big changeover to the 9mm auto from the revolver, many departments decided the .40 caliber was better and traded in their 9mm's for the same brand and type, only in .40 caliber.
The big wave of used police guns was in the late 80's early 90's when American law enforcement made the big switch from the revolver to the auto.
You could buy virtually new revolvers cheap, and some of the best gun deals ever were on S&W Model 66 and 686 revolvers.
The second big source is from departments that have a system of replacing guns that are considered to be getting near the end of their service life.
A department figures out how long a pistol will last on the average.
When their guns reach that point, they trade them all in.
Under this system, you'll find a mix of guns ranging from clapped-out near-junk to guns that are in virtually new condition.
Most police guns are "carried much, fired little".
They can be excellent buys.
A new police gun is shipped to the department, where in most cases, a department armorer checks the new guns out to insure there are no factory defects.
Any that have problems are either returned for correction, or are corrected by the armorer.
The gun is issued to an officer who is responsible for it maintenance.
Periodically, his gun is inspected to insure it's in good working order, and if a problem is detected, the gun is either repaired by the armorer, or returned to the factory.
The officer shoots the gun in qualification, which is usually just enough to break a gun in over it's period of use.
What all this means is, if the gun has any problems, they would have turned up either in inspections or in qualification shooting and been corrected.
The gun will have been fired enough to have been broken in, but probably not enough to cause any real wear.
True, some officers treat the gun abusively and these can be badly finish worn, with possible corrosion, damaged sights, and a battered exterior.
However, most of the ex-police guns are in excellent shooting shape with the wear you'd expect for a gun that was carried daily for a few years.
Bottom line is, if you can find one that's not too battered looking, chances are you're getting a properly broken in gun with all possible problems eliminated, cheap.
In this, buying a used police-issue pistol is no different than buying any used gun, with the added advantage that it's very unlikely anyone has disassembled the gun and tried a "Billy Bob" trigger job or other alterations.
Most departments make disassembling a service pistol a firing offense, so you see very few guns that aren't in factory-spec condition.