[dave4420]

In theory Congress can impeach him for it, but in practice there won’t be the votes to convict him. If there are ever, somehow, the votes to convict him, it’ll be for something worse.

After his term ends… I don’t know whether the law says that the insider trading exemption Congress has also applies to the Executive branch… but I’d fully expect the Supreme Court to say it does, if it got that far.

[Nursie]

Isn't he legally immune for anything done in the course of his duties?

You'd need pretty solid proof he was colluding with others to place trades, and not just running his tariff policies as he saw fit with advanced notice given to his inner circle as some sort of courtesy.

I'd be surprised if anything could be made to stick.

[actionfromafar]

Crucially, you need the proof, but you are not allowed to bring the proof to court!