I recently purchased and Sig P229 and can share my experience.
I didn't know about the website referenced above. While researching, I came across many police trade-ins. I really wanted a 9mm, though couldn't find any. My next choice was a .357 Sig. The ones I came across had Sig's DAK trigger and I know I don't want that.
Based on my sources, .40 S&W was the only choice. I had one option that had that police trade-ins. Those pistols had sig night sights, a picatinny rail or both, The vendor rated them good to very good.
I have had another provider selling the Sig certified pre-owned guns. Those guns go back to the Sig factory and one of their techs restores it to the specs of a new gun. The downside with these is that you didn't know if you would get night sights or a picatinny.
With the selection I had, I went with the certified pre-owned. It's really nice, shoots well and is very accurate. It has neither night sights or a picatinny. I would expect that the police trade-ins would have been nice as well. If the choice had been a used 9mm I would have gone with the used gun, especially if it had night sights and a picatinny. I would have gone with a used .357 sig with night sights over the certified pre-owned if that had been a choice.
From a pricing perspective, the certified pre owned was $510 and the used guns were mainly around $500.