What are you looking for?
There are a few ways to get "police trade in" firearms.
1. Some Depts/Agencies actually hold public auctions of traded in/seized firearms.
This normally doesnt do much for the dept, as these funds then flow into the general fund at county or city level.
This means the dept. loses control of said money, which means it gets spent on streetlights and raises for dog catchers.
2. Lots of guns are traded in to LE supply companies such as Craigs, Gulf States, GTs, and Hero-Gear (we have about a dozen bids out right now for this very thing)
What happens there is a dept will solicit bids to 2 or 3 companies, asking "how much credit will you give us for our junk"
The company with the best bid normally wins, and the dept. ends up with some nice credit to be used for things like; new duty guns, duty gear, uniforms, etc.
3. Here is where things get interesting.
Places like Craigs wont sell to the general public, so what do they do with the guns?
They mark them up, and sell them to gunshops or distributors, who then mark them up again and sell them at retail.
So, in theory (and generally speaking) MOST gun stores have access to used/traded in guns from police departments.
They order them from places like; CDNN, Craigs, Lew Horton, etc.
Very few shops actually have direct access to traded/seized firearms that come directly from the dept.