is there a way to filter searches by "days on the lot" at the dealership?

Wow! Seriously? That's your answer? Time on the lot does not correlate with decency/ condition of the car! Some dealers have stock that simply is not popular or too expensive for their market. The more time on the lot usually means better negotiating power as dealers need to pay back manufacturer credit. That is why I think searching by time on the lot is relative to me and I'm sure plenty of other buyers.

You seem to be missing Tom's point. When a car sits on a lot for a long time, it means other people have found reasons not to buy. They might be good reasons or bad reasons, but the perception of value IS value. On the other hand, if you are tolerant of whatever it is that turns others off -- the orange paint with two-tone blue and yellow naugahyde interior, for example -- then, yes, with a NEW car, you are in a great negotiating position. But if a used car won't sell, beware.