AutoTrader is popular in the US for cars, and they aggregate some dealer's listings. This would work in the US, but prices for different types vary widely based

AutoTrader is popular in the US for cars, and they aggregate some dealer's listings. This would work in the US, but prices for different types vary widely based on region. Using a Metropolitan Statistical Area would help. I once wrote something sorta like this for motorcycles, which are easier to compare due to fewer model options. My goal was to identify what brands or models held their value well (Hondas and Harleys mostly) and to automatically alert when one fell below the curve by X%. I never got much out of it, but it was a fun project.