Even buying the original Japanese release is kind of in a gray area as you're technically buying pre-owned at this point, so the original publisher isn't receiving your payment (they were 'paid' from the original sale).
That is what publishers wish, but the used market is perfectly legal. Neither importing (cross region) nor selling used software is illegal. Some publishers tried to push shady stuff like that into their
"You clicked on accept, so everything is valid!" EULA… and learned a valuable lesson the hard way:
Law > EULA
Microsoft got themselves a bloody nose in court by trying to pull off that stunt
(so they went ahead and made it harder to impossible to transfer licenses on technical level starting with Win 8.0)
Rightsholder gave out a license. This is (was) usually tied to the original medium (GBA cart in this case). You can sell the valid license without the rightsholder being able to do anything about it. Here in Germany I'm also allowed to create a backup copy (which cannot be forbidden by EULA although they try all the time) of any software "if this is needed to ensure future usability", which means the license won't expire if the original cart breaks.
This is exactly what I did: Import the Japanese GBA game, dump it with a DS Lite and apply the English patch. It is hard to find the original right now. For whatever reason eBay is tolerating bootlegs systematically (although forbidding in eBay terms of service and reports get ignored).
The "retro" gaming market being ruined by inflated prices is never going away, which is a limit on what you can and should buy legally. Although legal, I would advise against paying inflated prices for decades old "retro" stuff – it is supporting greedy people.
but imo you should never pay for piracy - which obviously includes 'reproduction' carts.
Fully agreed. Don't support commercial piracy. If the piracy market vanishes, they lose some arguments against homebrew/CFW. Supporting commercial piracy is a disservice to homebrew communities and it will be used as argument against free access on devices we payed for.