He will make plenty of “legit” money giving speeches, making appearances and writing books. I dont begrudge him this, after all how much has Dr. Carson made selling his book? I am sure he could get advances and loans based on his future earning potential.
The problems imo arise when you or your household or your ‘charity’ take money while you are sitting as a high government official like Secretary of State, or director of the FDA. Dr. Carson holds no office. It can get murky for Senators and Congressmen, esp when groups buy 1000s of copies of your book, but congressmen have only one vote with no discretionary powers. Agency heads and cabinet officials do have discretionary authority, so imo its more cut and dry that they should not be taking money on the side.