Contact the Department of Transportation – Motor Vehicle Department ( MVD) or go to their Web site in your State. If you plan on doing any financing or arranging financing you also need a license from the Office of the Consumer Credit Commissioner.
For a retail used car dealer, you must pay a license fee for dealer license application plus a fee per metal dealer plate desired (most small dealers only need a couple of these), obtain a car dealer surety bond, the bond amount is set differently by each State, or has an irrevocable bank letter of credit for the amount of the Surety Bond Required. For example in Texas the amount of the bond required is $25,000 for 2 years. In addition, you must own or lease property with an office (separate from and with no access to any residential living quarters on the property), at least 5 paved parking spaces, and a sign with six-inch letters. You must also have a working landline phone installed in the office, have a desk and at least two chairs, and there are other provisions to get your license application approved.
For a wholesale license, you must have an office, but need not have specific parking spaces for the sale of vehicles on-site. But with that license you can only sell to other wholesalers or dealers. I think it may also be permissible to sell to out-of-state retail buyers, other states do this, but you may wish to check with DPS before trying that.