'Refine By' menu shows an arrow on mouseover, but user can only click on words, not arrow or high-lighted area.
'Refine By' pop-out menus show a square, value as hyperlink, count. Square looks like a checkbox (with rounded corners) but clicking on square produces no result (this user expected a checkbox response, & ability to select multiple models, colors, etc.)
Change of other filters does not require a 'go' button, but change of search radius does.
Results appear to have a wide radius, but pull-down for location does not show what default/pre-selected radius is (by experimentation, 10 miles).
Generally, the user should be able to tell what filters are currently active, and what their values are.
Clear Price filter action appeared to clear all filters.
US states and placenames with multi-word names should have all elements capitalized: s/New mexico/New Mexico/ s/San luis obispo/San Luis Obispo/
Price filter should allow either end-point to be absent.
Mileage should allow arbitrary range end-points, like price.
Year should allow a range, or checkboxes.
Consider having Refine by Make hide the less popular makes behind a 'more' button. So, by default display top N makes & 'more', 'more' displays top N2 (or all) makes & 'more', top N4 or all, top N*8 or all, etc. (otherwise the menu may grow to many dozens of obscure makes)
Support 'open in a new tab' on the hyperlinks show below the first search box. Searching displayed 3 links with counts, but displays nothing - sigh. (perhaps the back-end function is not yet implemented?)
Again, impressive data.