You don't need to be registered. I have a merchant account simply because it looked like the best fit for what I planned on doing with the account. The number of transactions, limits and fees are all considerations.

If you are a US citizen, there may be some tax implications as well - whether a business pays the taxes or an individual. As I am not in the US, I cant be sure how it is handled though.

Just look through the various account types and find the most suitable one.