[dragonwriter]

> If he can set 25% tariff on everything imported from China, what's stopping him from setting 2500% tariff on everything?

(1) the absence of any statute granting him the authority to impose export tariffs, and

(2) the fact that any statute which did exist purporting to grant him such authority would be invalid under the Export Clause, which explicitly prohibits export taxes and duties.

OTOH, he could prohibit pretty much any trade he chose with China under the International Emergency Economic Powers Act (the main law under which Executive-declared sanctions are instituted.) He just can't tax exports.