Advanced Questions about Sponsored Posts

Many blogs offer sponsored posts.This definitely is not allowed from Google, but how actually they recognize that?

Example: If there is a blog who sells sponsored posts but they also accept guest posts, what would happen, if we get links from their guest posts? This site sells links, but we don't pay for links because we get guest posts, what would happen?

How Google know that the links are sponsored?Manually or algorithmically?

4 questions:
What if the sponsored/guest posts are with nofollow/dofollow links?

What if the site has / doesn't have information that they sell links? Is this a factor that we have to pay attention to? For example I found a great blog, I contacted them and they told me that they sell sponsored posts, but there were not information on their blogs about that? Am I safe to buy links?

Then the next questions is what if I buy links, but I don't keep them in their sponsored sections?

Does if affect the fact that the blog has category "sponsored" and we have links from / not from this section?

Does it affect that the post we buy ranks with priority in their blogs, is this a footprint in the algorothm?

Officially Google will tell that they penalize link sellers, which is obviously not true. There are many cases that link buyers are penalized.

Please let me know.