Like many high net worth individuals, his net worth seems very abstract. It's partly based on his ability to generate future income.
There's the retirement accounts and piles of cash, of course, but those are the easy ones to count. Then there's the real estate, both residential and commercial, which is harder to value. And lastly there's the business, the corporate entity that contain the MMM blog and the construction work and the hq building and everything else. Like the real estate, the business is worth whatever someone will pay for it, and what they will pay for it is typically some multiple of it's annual earnings or expected future earnings.
Based on the numbers he's posted thus far, I estimate the the business is worth more than all of his other assets combined. If he wanted to sell it, there are lots of blog-buying businesses out there that offer successful bloggers large up-front sums to take over ownership of the blog, usually with some conditions like keeping the deal secret, keeping the founder contributing on a semi-regular basis, and monetizing the pages for new revenue. You can go read up on the history of Get Rich Slowly for an example of how that works, because that guy cashed a seven figure check and didn't tell anyone, but guest posts and ads started appearing on the site as he moved on to other things. Perhaps relevantly, that whole process happened roughly concurrently with his divorce from his wife.