In the for sale sections have extra boxes (like the subject section) where you list more detailed information that can then be specifically searched for.

Eg:

Car make: Ford / Renault etc
Model: Sierra / Escort etc
Type: Cosworth / 1.6 Ghia etc
Cost: £3000

Then set up the search feature so you can list all Fords, all Sierras or all Cosworths or all cars between x and y value etc.