I have physical games that are Japanese only regarding language support, if it’s a PS5 disc or Switch cartridge: the covers are of course written in Japanese & the region code for it is JPN (even if consoles are region free, why are language locks still a thing regarding the software as there’s no ENG?)
But, can you still trade them despite accepting this caveat: locked under a language barrier as the UI’s in game cannot be reverted to English. (Also, the classifications use CERO framework rather than ESRB). Like a CERO D (17才以上対象) is akin to a ESRB M17 & CERO C (15才以上対象) is similar to ESRB Teen.
It can be insanely hard to find any demand for them if you’re talking about trading locally rather than selling online and shipping. That’s probably the biggest real obstacle.
In terms of legal or regulatory issues, no, nothing is stopping you unless maybe you’re dealing in porn-focused games in a state with strict anti-obscenity laws. Don’t lay the 18+ doujin soft out on the Sunday flea market table in front of your local superior court.