This answer is preliminary – see my comment asking for clarification.
It really depends on what you want. Do you want the product page …
- … not to be available in Google's index any more?
- … not to be discoverable on the website (e.g. through a product search page)?
- … not to be accessible by direct URL?
If the answer is yes to all three, simply delete the product page or disable it. It won't show up in any search results on the website anymore and the direct URL will lead to a 404, so it will quickly vanish from Google's index.
If the answer is more complicated, you can take different measures depending on the intended result:
- The page should be removed from Google's index: Simply add a metatag to your sold product pages (
<meta name="robots" content="noindex, nofollow">) and remove it from the sitemap. It will vanish from Google's search results in time.
- The page should not be discovable on the website (but still accessible via Google or direct link): Modify your product search pages / feeds to not display sold items anymore (you can determine the status based on stock, a lightswitch field, or whatever makes sense for your business process).
- The page should rank lower in Google search results:
Keep it in the sitemap, but give it a very low priority (like Edit: As pointed out by RitterKnight in the comments, Google ignores the priority completely. Pretty annoying, but it might still be worth including the priority for other search engines that don't ignore it. Guess it's all or nothing with Google.
0.1). This will signal to Google that you don't consider this page important, so it will show up less often.
Mix and match the above as required. The simplest approach would be to disable the page entirely. Again, it depends a bit on what you actually want ot achieve.