Hi,
This is a great article (on the Moz Blog) on how to handle expired content. It mentions pros and cons of the different methods, and though it's from 2012 I think it's still highly relevant. In particular it talks through adding 301 redirects with custom messages, so that you can explain to your visitors that the property has been sold already, and redirect them to a search page, list of nearby properties, or similar. I'd make this really clear, with a bold message of 'Property sold!' or something similar, to avoid quick bounces.
I would personally avoid 404s or 410s, as even if your resulting page is helpful, some users are likely to bounce from this page without even reading the content and realising it's helpful to them. Also, of course, it completely stops the link value of the previous page.
I'd try to get the best of all worlds, and aim for the most user-friendly 301s possible as explained above.
Hope this helps!
Zoe