To me it seems like amazon has a limit up to which it will alow you to buy obviously from a different country. After that you get locked out, but even after a single verification, it will kinda give up. Same happened to me.
Personally I prefer amazon for a couple of reasons:
- I’m already knee deep in their ecosystem, I have a kindle and about 100 odd books purchased through their store.
- They allow you to actually download the books properly, and if you have the know-how (or you google for 5 minutes), you can even remove the drm protection and use the books as ebooks or pdfs (really useful if you for example want ot run it through Mokuro for an easier reading experience, or if you want to upscale the manga or whatever).
- Their reading solutions are just way beyond that of for example bookwalker. Bookwalker reloads the current page very often. Just thinking about exiting the app for a second causes it to reload. And this does take a long time. Kindles are nice too, for books they provide a pretty decent dictionary system, that can even deconjugate somewhat.
They have a couple of disadvantages too, main one, is that the only way to read actual proper books is to either use a kindle, or either the desktop or phone app (or you can remove the drm and just not care about this).l