Trump could have made a lot of money by selling stocks before tweeting about imposing tariffs on China and then buying more of them back when the price of the stocks has dropped. Would this fall under the ban on insider trading, or does insider trading only apply to confidential information about companies?
There is no statutory definition of insider trading, and the question of who is included is answered by the SEC. It includes "Government employees who traded based on confidential information they learned because of their employment with the government". Under 5 USC 2105, POTUS is an "employee", though that is w.r.t. Title 5 and insider trading laws are under Title 15. But as it happens, the Title 15 definition of "executive branch employee" assigns the Title 5 definition of "employee" to "executive branch employee" and explicitly lists POTUS.
Pub. L. No. 112-105, §§ 9(b)(1) explicitly says
Executive branch employees, judicial officers, and judicial employees are not exempt from the insider trading prohibitions arising under the securities laws, including section 10(b) of the Securities Exchange Act of 1934 and Rule 10b–5 thereunder.
Insider trading laws are not limited to "information about company X", they are framed in terms of non-public information, which could include information about a country: "no executive branch employee may use nonpublic information derived from such person’s position as an executive branch employee or gained from the performance of such person’s official responsibilities as a means for making a private profit".
I take the relevant part of your question to be whether a president can break the law, taking it as a given that the stock manipulation you describe is illegal if proven.
The short answer is yes, the president can, has, and will again break the law. The president is above the law.
There are basically three ways the president can be removed from office:
- forcibly by assassination, kidnapping, and similar violent actions;
- by impeachment by the senate for crimes and misdemeanors, but there is no precedent for that;
- by an application of the 25th amendment, if the president is incapacitated by a mental or physical illness; but there is no precedent for that.
None of these are realistic threats today to keep President Trump in check, suggesting that the crime you describe CAN be committed without sanction.