Im a little confused with the concept, but what is selling a blog post? Also, how much can they be sold for?
It's when you write a favoring post for pay, like product reviews or promoting a company. It doesn't pay that well, and it's not beneficial to your SEO. Paid posts (if you are found out) are penalized by SE's. I think there is still a company called Pay Per Post that does it, but you need go PR and traffic to get the good rates.
you would review an item given by a company so they could promote their product in your blog. usually done at gadget blogs with a lot of readers. sometimes they gave you the gadget for free. isn't that nice?
Make it easy, the blog post buyer will pay you for writing review about their product or website and post it at your blog. The price is depend on your blog PR and/or traffic