Technically you are selling to the whole world not just the USA. You also can trade in other stuff beside games which would mean everyone who sells in itch.io would be consider to be engaged in a trade. However this is not the correct answer because each country has different laws so what you consider 'engaged in a trade' wont legally translate to another country.
This is why itch.io is Merchant of Record because they will handle this for you. The only thing you need to do is file the correct tax files your country requested. You maybe consider a sole proprietorship or business depending on what country you live.
