Lets imagine that the word is along the lines of Fotolia. It’s a made up word, and it’s brandable.

Imagine that someone in Australia is using Fotolia as the name of their personal photography site/studio.
Someone else in Europe has registered it as a LTD company (not sure for what purpose).
Another person in another part of the world (possibly US) is using the name as a small ebay store to sell photography equipment.

The name is not trademarked. The .com (amongst others) is available.

Could someone else (me) use the name as.. let's say a multi-user portfolio site. Aimed at USA users, but could become used worldwide. So, a slightly different purpose but within the same subject area.

Am I likely to run into any legal trouble with doing something like that? Could I file a (USA) trademark claim myself or just stick ™ on it, without them trying to fight me for it?

Can I file a US trademark despite not actually being/living in the US?

Any advice would be lovely.