Maybe my understanding is wrong but the tariff is borne by the buyer / importer. So regardless the pricing, if you are buying from Japan as a person in the US you will need to pay higher fees getting the cards to you in the US.
If you are trying to sell your cards in Japan, those should be going for market value and not higher price because the tarrifs should have no impact. Only time will tell if the buyers from US to Japan will reduce because of these tarrifs, which may lead to cheaper or more competitive pricing from Japanese sellers.
Cheers!