What violates a copyright license?

1) Any time you reproduce a trademarked or copyrighted image, you are violating intellectual property and could be sued--it does not matter that a customer asked you to do this; that is not a defense, and you could still be liable. Assume all major characters (e.g. superheros, disney characters, cartoon characters, etc.) are protected by copyright and/or trademark.
2) You *should* set up an LLC and conduct business under it: that will help protect your personal assets from most (but not all; no protection is perfect) business debts, judgments, or obligations (e.g. if the business is sued by a dissatisfied customer, or defaults on a business loan, so long as you did not personally guaranty the loan).
3) You need to register in your state as doing business. If you do set up an LLC in your state, that will also take care of registering to do business.
4) If you do set up an LLC, then go to the IRS's website and get an EIN (employer ID number, a/k/a tax ID number): this is basically a social security number for an LLC (or corporation).
5) Check with your state's department of taxation to see if you have to collect sales tax on what you sell and, if so, a) what the tax rate is, and b) where and how to send it in. Be aware that sales tax is a "fiduciary" tax: that is, you would be *personally* liable for any shortfalls or nonpayments or noncollection, so if you have to collect sales tax, be very careful to do it right.
6) Even if customers do not come to your house, you will have to notify your homeowner's insurance that you are conducting a business at home and the nature of it, and will likely to have to buy other or supplemental insurance; if you don't and you put in a claim, the insurer could potentially deny a claim if the business is any way implicated (e.g. a tool you use starts a fire), since you failed to disclose a non-residential use of the home.