He asserts, and many lawyers agree, there is nothing illegal about this, though there is a clause in the Constitution ("emoluments" -- Merriam Webster's the word of the moment) that might serve as some check -- if Congress decided it wanted to impeach him for accepting profits from selling the policy of the United States to foreigners. It's hard to see that boot-licking bunch even trying.
A Wall Street Journal article (paywall, sorry) outlines how presidents became exempt from the restraints on other officeholders under conflict of interest law. (Hint: this involved protecting slave owners.)
Matt Yglesias has explained how plunder by a political leader can become "systemic corruption" which is itself a form of governance.