True story yes, but those aren't common outside of Gen2 police trade ins. A $400 Gen 3 Glock is an exception - usually a newbie who doesn't know better or who needs quick cash. That's why, as you said, you have to be quick.
I have to add this. I have seen this "Glock prices" thread many times, and I make a mental list of those members who either complain about the prices or say "I would never pay more than x for a used Glock". Inevitably in the future they end up listing a Glock for sale, and its priced right up there with the rest, or it's listed trade only so they can mask the fact that they are trying to get 550 worth out of it.
I'm not one of them. I sold a G22 for $325 2 months ago and that was after i brought it to glock to have gone over. I won't support ridiculous used pricing - i'll just buy new.