This is a good casual study, but only casual at best. Did you factor in the sales numbers for vehicles? That is, if 150 civics were sold in an area and there are 50 used cars it does not mean the same thing as 10000 cars sold new and 50 for sale. Also, if you're not going to use a random sample you should probably weight by some city size factor. By not using a random sample there's a much higher chance of user designed bias.
I don't mean to bash, but when you put something out for the world to see it will be critiqued. Perhaps everyone here reading this saw those flaws, but perhaps they went unnoticed by some and deserve to be pointed out for their sake.