A custom tariff isn’t a tax, it’s a fee. Also, what they are asking to do is register future imports. Not past imports. You are right that they can’t make people give back their cars, or impose ‘tax’ on a purchase. They can however say “if you move an EV produced in China into this block you will have to pay a fee. We’ll tell you how much later” - it is your decision to accept such uncertainty. If you have already imported your EV cars, hooray, you are not affected.

To put it another way this isn’t about purchases, it’s about movement of goods. If you import your own stuff, you own it before you move it. You don’t pay customs on something you bought abroad, if it stays where it is. Its the moving that matters. Similarly if the sale is after moving the goods, there’s still no part of it that is affected by the tariff. You pay a tariff to import something whether you manage to sell it on later or not.

Reading a slightly less shit story will explain this for you.

https://fortune.com/2024/03/07/eu-nears-hitting-chinese-evs-with-additional-tariffs/