How does used car pricing work?

My wife and I are in the market for a used car and I’d like to get some understanding of how used cars are priced so I (hopefully) pay a fair price instead of getting fleeced.

Edit1 - not in the US but in a country with a relatively mature used car market where the ratio of used cars sold to new cars is 1.5:1. In comparison, the US has a ratio of 4:1.