Several of your questions have been answered by others. Being a Realtor means you are a member or the National Association of Realtors (NAR). You can have your license and sell real estate without being a member, but most real estate firms require it. What you should check out is joining ICREA, see http://www.worldproperties.com/ , FIABCI http://fiabci.org/ as well as NAR and getting your Certified International Property Specialist designation. You can do this in Europe as well as the US and I know Bulgaria is offering the complete course at least twice this year. Any questions, drop me a note at email@example.com.
To answer Jesse's first question, I guess I would specialize in residential sales allthough I really like the commercial estate market also. In Holland we can do this both if we want to.
To Aileen, what does it take to start working for a firm like the one (CB) you work for, what, next to a very good motivation, is needed to join forces there? Excuse me my ignorance but what is the difference between a Realtor and a real estate agent, does that have to do with national recognition of your classes taken? Thank you for providing me with this handy link!
ps. I have no problem with any of you contacting me directly through Trulia.
Jesse is correct. Each area requires you to obtain separate licenses. Regarding real estate sales, specifically, you have to take a required number of classes. These classes require that you be present in NYC. I suggest you read the requirements from the NY State Department of Licensing. I've provided the link below.
Additionally, I suggest you relocate to NYC, full-time (at least in the beginning) to learn what it's like doing business here. For real estate, at least, being a Realtor (not just a real estate agent) is... different from being a Realtor in Buffalo. In Manhattan there are tens of thousands of co-ops and condos, new developments, pre-war conversions. You can learn the real estate landscape only by being here. If your intent is to refer your European clients to NYC, then you can reach out to a local brokerage and arrange a referral fee for each of your clients that purchase real estate. Coldwell Banker (my broker) has global offices and we regularly work with those offices.
If you have any additional questions, Trulia is a great resource for you, but I suggest you contact a professional in each of the areas you mentioned, so you can have an in-depth discussion of the pros and cons. Stating that, please feel free to contact me directly.