RetireJapan

Any suggestions for where a 20-something Japanese citizen should start?

Books/wepages etc. in Japanese or English, both are fine.

He wants to legitimize his growing online resale/grey market business and perhaps launch an app in the future.

There is so much out there it is hard to know where to begin.

Go and talk to the local 商工会. They usually have free consultations, info on grants, help with paperwork, etc.

Fukuoka for instance has a Startup Cafe and some mentors that can help people launch their business: https://startup.fukuoka.jp/

I think other prefectures also have some similar initiatives but you'd just have to check depending where you live.

It couldn't hurt to work on bookkeeping. My wife did the level 3 test as prep for her tutoring biz, to learn about the tax filing and accounting used for that--she does it on her own rather than paying an accountant. Double-entry bookkeeping is 複式簿記.

The test is 日商簿記検定. She studied for and passed level 3, but studied independently (books/youtube). She says that students who went to a commercial high school would have no trouble with level 3 or 2, but as a language teacher she didn't have that background.

I was overseas for a couple weeks and forgot I asked this question before leaving.

Thank you all for the advice. I passed it on.