There is an influx of agency trade ins on the market right now, so there are some good deals out there.
Regarding personal sales, I think most come from consumers that buy them and figure out it isn't the right pistol for them or you get people that have had them awhile and want to consolidate their pistols to something different.
For instance I shoot 1911s better than any other pistol and Glock is a very close second. After that it goes down in various degrees, but Sig is a couple of pistols down. I have a German P226 sitting in my safe doing basically nothing because, while it's an excellent pistol, it just doesn't work as well for me. A friend of mine, who also happens to have once been an excellent competition pistol shooter, once told me to find what I shoot best and master that pistol and I have more or less decided to take his advice; and since the Sig isn't a pistol I shoot as well as the others, it's time for it to go to someone that will make more use of it. Nothing wrong with it, just not right for me.