ESRB is not a government entity. It's a US game industry rating board.

As long as whoever you are trading to accepts that it is Japanese only, there should be no problems at all. My local game shops have a section for imports that people trade-in.

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.

Forget trading them, there are stores where you can outright BUY them.

Depends on the shop. You might also get a lot less for it, especially if it requires a region matching console; its going to sit on store shelves for a while, so the store owner might feel like the hassle isn't worth the low cost unless its a rare game or something.

On why only have one language, maybe size, licencing for a given translation having different rules than the rest, focus in a given market, position of the planets... Could be N reasons.

On if you can trade games, only vaguely familiar with Canadian and American laws, but afaik, nothing should be stopping you? Maybe aside from a game being explicitly forbidden (dunno if either have such a list), or if they break some broader law not necessarily related to games.

Also more broadly, among people with Japanese as a second language, native Japanese speakers living in the two countries, collectors, resellers and import sales, I'd say you have a non-zero chance of finding someone interested.

And about age-rating boards, I don't think I've seen them have Executive/Legislative powers in either countries, only as a recommendation/guideline measure. May be good to confirm on that though.