How to create a business to consumer marketplace?

Hello guys, I from a small country(Austria), where a lot of potential is for new startups. A lot of marketplaces which are created in the US and become successfull there are not even thought about here or are created years later. I plan to focus my startup expertise on the creation of business to consumer marketplaces because I see a huge potential here. However, I am completely unsure where to start? Should I just bring another US startup to europe? Should I exploit my own niche?(How to find the profitable niche?) I really appreciate your answer!