I'm from Germany and want to run small events (around 50 participants) in the United States, which involve the sale of tickets (probably online) and payment to contractors which contribute to the event.
Do I need to incorporate in the US in order to do this? Or could I do it as a private individual or with a German company?
If you are running it as a private individual, then you don't. In that situation your operation would be recognized as a foreign sole proprietorship. See the discussion here: http://www.askmehelpdesk.com/advice/t-435759.html Of course, incorporating helps protect your personal assets from liability, so keep that under consideration.
If you are running it as a German company, then you will need to foreign register/qualify the company to do business in the state you're operating in - different states have different requirements for foreign registration.
I don't see you needing to incorporate in the United States unless you want to do this on a permanent basis. If you do I would suggest creating an entity in the US which might be a wholly owned subsidiary of your German company(the exact set up will need lawyers and accountants to be consulted). But a company in Germany can sell stuff to the US provided import/export laws are obeyed and you may need to report what you make to the IRS using Form 1042 or Form 1120-F if you are doing this as a corporation.
The exact forms taxes and legal implications of you doing this would need to have lawyer and accountant involvement.