jimr451 - 2:23 pm on Feb 7, 2013 (gmt 0)
When you sell a car - do you delete that entry? Do you have any example URL's of "sold cars" just to verify they are actually deleted?
It's possible people are getting "cached" copies of old pages from a search engine. They might click a link, and get a "page not found, but we can show you a cached copy" - so that might be both problems rolled into one.
If you find this is happening, you might consider updating "sold" car pages with a "SOLD" but here are related cars we still have (and link to the relevant category page). That way the old SEO juice sticks around, and still gets people into your site.