Not selling internationally will not affect your ratings.
I did register with that German site. You can do it also. Just fill out the form. They ask about the EU VAT tax number, you can use ebay's. There are two more numbers they ask for, I just typed in not available and it went through.
Then one is supposed to register with one of the government approved trash collectors and indicate the amount of packing material I will be using to mail stuff to Germany. It was not clear whether one does an estimate the annual volume of packing material or after the end of year. Based on that info, that trash collector site will calculate how much you have to pay them to process the packing material the german recipient of your goods will be disposing.
I rarely sell to Germany, one thing I figured, that if you mail something, make sure there are no logos (like ebay) because then they treat this packing material differently and you will pay more . Actually, there are about three pages!!! of explanation what is considered packing and then how to classify different types of packing, and what is not. E.g. If you mail a cardboard game in a box, and the cover of the box is used to play the game, then the box ix not considered trash. Otherwise, it is trash. Lot of nuances. Not that easy. Totally crazy stuff.
Also, it is only the question of time before bureaucrats figure out how to impose garbage collection taxes on the content, because sooner or later, the content will also find itself in trash.
Just more complication that hinder free trade.
