Things needed to have free web app available in the US

This is my very first post here. For quite some time I am a passive/occasional reader, but the time has come. :)

I am about to launch beta version of the app that I have been working on for couple months now. Still have some minor bugs to take care about and to host it somewhere.

In addition to that, I am also starting to work on landing page and marketing plan, supported by a friend who is a professional in that area as I have engineering background and no experience or knowledge there.

The app is a marketplace for listing certain kind of goods that need transportation. Somehow similar things do exist, but I see my chance in different pricing model and in different approach - instead of having closed community where you first have to go through complex and slow registration process, I would have everything "open" and all listings visible to non-registered users. You would have to register only in order to create a new listing or give transportation offer for the existing one.

I had plan to start with a completely free version. I anyway don't have payment mechanism implemented at the moment and I would be more than happy to validate the idea and get some feedback as soon as possible.

So coming to the question... I am planning to target US market only for the beginning. I have the impression that people over there are the most open for new things. What do I need from the legal side in order to be safe with this? Do I need to have a registered company? Does it have to be in the US?