I can think of a few ways (not sure all within TOS), not sure if helpful but still worth a shot (maybe): Build a site for people wanting to buy a car and know what's the avg asking price of models with the same characteristics. For example: say I want to buy a Freelander for max 10K. I'll go and check all freelanders and quickly notice that most in that range are from the year 2010 and 2012. Kilometers differ, guaran…
The advantage of this, if you pick something common with hundreds of listings, is you could easily derive a relationship between the two variables and see if a given ad was above or below average.
I never had any idea where to begin marketing it to other people though. I suppose if I knew someone who provided a car buying service...