EShop account?

I bought a used Nintendo 3ds. I then formatted it. When starting the Nintendo eShop I was then surprised that I had to agree only the standard terms and conditions but did not create an account at all. Nevertheless, I'm in the eShop and there's also a point delete account, why I would then enter the associated serial number. Where am I logged in after formatting? Is virtually every Nintendo a separate account? Or is it the previous owner? Do not want to deposit money into a foreign account now. And if I delete the account, I can then create a new or is the serial number unusable then?

I'm not quite sure why after formatting there's still a Nintendo account on the 3DS, but I suspect that it is the previous owner's account. Maybe you should contact the previous owner to clarify the matter.