He just confirmed what I knew already, and I am going to enumerate some rummors for other game developers:
(keep in mind that this is for US, and the legislation might be different depending on the state)
1. You need a company if you make money.
Bullshit. You don't need anything.
If you operate under your name, and you are the only owner of a game (or if it is a familly business) you can operate as a sole proprietor, and nothing else is required.
2. You need to register with the state.
Not really. Only if you operate under a fictitious name. If you use your real name for the payments, there is no need to register.
3. You need a federal tax ID.
Not if you are a sole proprietor. You can use your social security number.
4. You are not considered a business unless you register with the state.
Not quite. Acording to the IRS, you are a business as long as you start something with the purpose of making money, wether or not you actually make money.
On other news, I am currently working at implementing a guild owned and operated merchant NPC, so that the guilds will be able to have their own ingame shop, with them in control of the items the NPC buys and sells, prices, quantities, etc.