ASG04; Laws vary from state to state, but here in California you need to....
Get fingerprinted, and pass the live-scan check
Get bonded by an ins. co. for $50K, or post the 50K in a savings acct, with the Dept of Motor Vehicles listed as loss-payee.
Take a 8 hour course that updates you on current laws affecting your business, then take and pass a test, of about 60 questions.
Get a proper zoned location with a sign posted above it visible from 100 feet. Post your hours on the door, and maintain an office accessible from the street without going thru another business.
Obtain a city business license or county, if in an un-incorporated locale.
Obtain a sellers permit from the state board of equalization. (you may be required to post another bond for this permit, depending on your credit, backround, experience, and projected monthly sales).
Now, after that is all approved, you will be given a dealers permit, and you can start buying and selling legally. Your dealer license plates arrive about 90 days later.
You still need to apply to do business at auctions, and they will screen your financials to see if they want to accept your checks, or put you on a "cash-only" basis. Some dealers can only sell at auctions, their credit, or banking history is so bad. Others have limits of how many dollars they can spend per week at each sale.
Thats about it.