Some steps for you to consider:
Arrange for an independent inspection before making the trip. Find an unbiased third-party auto mechanic in the area with no connection to your seller. Some nationwide companies, such as Carchex, contract with local mechanics to provide pre-purchase inspections for a fee. Use online Yellow Pages and other resources to directly deal with a mechanic in the area. Have the overall cosmetic and mechanical condition of the vehicle inspected to make sure it is a good buy. No used vehicle is perfect, but it is better to know about any defects beforehand; you can use them as a negotiation tool and to help you weed out problem vehicles.
Negotiate and agree upon a final price and terms before making the trip. Do as much of this over the phone or via fax. To protect yourself I'd look into drawing up a bill of sale that is signed by both you and the seller, and notarized as well. Depending on price I'd pay via USPS Money Order, so the weight of the U.S. government is behind you in case of any fraud. There may be a limit to have much you can pay via money order, so you'll have to look into that.
Contact your local motor vehicle office to determine the best way to handle your tax, title and license. Although you will be purchasing the car out of state, it still must be registered in your jurisdiction. Find out if any special documentation is needed to register a vehicle from out of state. If you are financing the vehicle, you need to make sure the seller or bank has the information for registering a vehicle in your state. Verify the correct sales tax is being paid. If not, you could be penalized, not the seller.
Make arrangements to pick up or take delivery of the vehicle. Personally I'd make the trip across country to take a test drive of the vehicle, even if you have ordered an independent mechanical inspection. Make sure you are satisfied with it before signing paperwork and taking delivery. If you are trading your vehicle, you can probably make the trip alone. If not, someone will have to come with you to drive one of the cars back. Having a vehicle shipped to you is possible, but it's not usually recommended to buy a used vehicle completely sight unseen.
Hope that helps.
