I like the visualization. This could be a great way to visualize if you're getting a good deal or getting ripped off.

Four things:

1. I don't see a way to change my location. Without that, the tool is essentially useless to anyone but you.

2. It's not obvious what the colors or sizes of the dots represent.

3. Have you considered superimposing Kelly Blue Book info? That could add some interesting trend lines.

4. A blog post analyzing how well different brands retain their value could be a great way to drive traffic. I imagine their might actually be some money in adwords around used cars.