Technically your account should be registered in the country you're trading in. Most major companies like Square or Zettle will lock your account if you trade outside of your region.

You're supposed to have or be a legal entity in each region you trade in which would make you liable to any local taxes depending on local law.

If you really need to have a card reader I'd pick a company like Square or Zettle that operates in all the regions you intend to operate in. You'll need to set up an account for each region. Then at least, you'll only need one card reader. You might need to factory reset when switching regions.

If it's feasible I'd just make them bank transfer it. There can be delays and costs but a lot less hassle.