A possible explanation is that bank transitions between countries often take far longer times. While eBay/PayPal handle some details, it is still the banking system at work underneath it all. Some countries have laws, and beyond that various institutions require different length holds to check/electronic transfers. 21 days is pretty long but not unheard of. As I learned a few years back, even for certain transfer entirely within my country (US) and even when the check “clears” and appears your account, the bank itself may choose to put a hold on the funds for as long as a month to insure the validity of the check, especially if it is a large paycheck or something. I would guess that there are certain kinds of shell game style frauds and can be perpetrated based on time delays, etc that they are guarding against.
Anyway, I highly suspect the policy has very little to do with the item actually changing hands, and instead is related to the fact that an overseas transfer of funds occurred. They might haves either a policy or an algorithm that decided for a certain transfer amounts, seller history, and point to point details that they will put a hold on things for “security”. This is not to say it is reasonable, nor that they aren’t possibly motivated by earning gains over and above the “security” issue. At the least they should issue a notice of this as a warning before the transaction is finalized. But as far a international transactions go, you are somewhat limited as to your ultimate rights and expectations. But the transaction was super documented so you will have everything you will need to sue them in the worst case senario.
I wouldn’t be surprised if there is also a classic catch22 type issue here - if the algorithm/manager receive lots of inquiries/complaints about how long it is taking then that would make them LESS likely to process the whole transaction quickly.
Weirdly, even though Japan is one of the most technologically sophisticated countries in the world, and an ally, etc. almost any time I have purchased things from there using eBay or any other online systems there were always many more issues and restrictions than when ordered from EU countries, especially from the UK. I don’t know the ultimate reasons for this, but that has been my experience many times.
Ultimately I would tend to blame the large and constantly growing level of Internet financial fraud as the ultimate source and not “getting too big for their britches” which happened a long time ago.