Selected answers from the Dumb SEO Questions G+ community.
Christina S: I`d respond with a 404 with a custom 404 pages that explains why the page is missing. Ensure there are no more links to it anywhere on the site either.
James Cosgrove: If you are talking about how to handle the pages while the cars are listed, I would not suggest using no follow or no index, the page should be indexed and followed while live. Once the car is sold the page for the sold car should then be 301 redirected back to your listing page or the search page. Ideally I would redirect back to a search result for a similar vehicle. Using a 301 redirect passes any link juice back to the page it is redirected.
Christina S: Using a 301 redirect passes any link juice back to the page it is redirected and that`s not very truthful since it`s not the same vehicle that was searched for. That`s why I recommend 404 with a custom 404 page with suggested similar active pages listed.
James Cosgrove: + thank you for correcting me. A 404 would be best/easiest. A 301 would actually be best only if you had a similar vehicle to redirect to and that would take some work so not the easiest solution. The 404 is best for Google and the user when you create helpful 404 pages that give helpful options.
Christina S: In that case I`d question why there were even multiple pages for the similar vehicles? I`d expect ONE page with details listed for the several vehicles that belong. As one of the vehicles is sold it is flagged that way on that ONE page.
James Cosgrove: On a custom 404 page you could indicate "This vehicle is sold. Please visit one of the following links to search our existing inventory". A custom 404 would resolve the concern expressed by Christina. The problem in my mind is all SEO value that vehicle detail page may have gained if any is lost and all links to that page become meaningless. So from the sense of helping to build future SEO value a 404 strategy puts a lifespan on links to specific vehicles. This makes a 301 strategy appealing even if it is not the ideal customer experience and if it is not the text book use for a 301. + not sure if any of this has helped. Hopefully we have given you a sense of the different options and how those options would impact you. Cheers!
Christina S: Oh but a 404 does say the page you requested does not exist. It`s up to you to make the custom 404 page as useful as possible. But of course a 410 is as good. A caveat: it`s a permanent condition so if ever you reuse the same url it will be impossible to get it indexed (at least in theory). So best to stick with 404. Google treats them the same way in any case. I see no SEO value in misleading the searcher with a 301 redirection.