Do search engines know which post is the newest?

Google search results are not ordered by date, but by query relevance, Pagerank, and other factors.

You are describing the function of an RSS feed – not a search query.

So what do I do to make sure when someone does a search for availability that they do not get an older post showing it is not in stock when in fact I have a post stating it actually is in stock?

Here is a good place for Google webmaster info like removing pages, caching, and snippets:

https://www.google.com/support/webmasters/