What are they actually selling?

Have seen some sellers offer; publish guest post on Linkedin with permanent backlink DA97 PA96

Is this like a normal social media post created on sellers own linkedin profile or company page?? Where they offer article writing and backlink placement?

Or is it like they actually have backend access to Linkedin.com and can post under their blogs/news section?

In linkedin you get the option for creating articles. Its different than social media posts.
Check it out:

Not sure about the link though, I know it's no follow if you just publish article with a link.
Also there is linkedin pulse. Find out about what's going with that in 2024, this is a little bit older video: