Can you trust a NAV blog?

So, the other day I got an email:

We noticed that you run a nice blog at hougaard.com. ******** would like to sponsor a post on your blog. We are willing to pay a onetime fee to insert a marketing message in one of your existing posts. An example message is: “Two useful technologies for working from anywhere – Cloud Desktop and Cloud SharePoint.”, with two dofollow links going to our websites.

I replied to them:

My blog is usually completely advertising free, so how much is a one-time fee?
How about me doing a review of some of your offerings instead?

They did not seems to like me reviewing their offering, that part got completely ignored:

I have been authorized a maximum of $40 USD for two permanent links on existing posts as per site/post stats. Please let me know if that can work out this time. you can send me your PayPal ID once you integrated the message and I will get this close today itself.
Post that we like:
https://www.hougaard.com/setup-reverse-dns-entry-for-a-server-in-azure/

I respectfully declined they offer.  But this got me thinking, how many blogs do sell advertising space on old blog posts to boost somebody’s PageRank? Have any of you accepted this? What’s your opinon on stuff like this?

What is acceptable advertising on a blog like mine?