I bought my car in California and brought it home to Canada. You MUST have the title of the car for the buyer, without the title he/she will not be cleared through the US export office at the border, believe me I know, been there done that. As was posted earlier the buyer must send info to the export office at the border at least three business days before the vehicle is exported, it can be faxed. A visit to the US border control website will inform the buyer of all the info that must be sent and where. When they get to the border they must have the original paperwork with them. It's an easy process as long as the correct steps are taken.

My biggest hurdle was at the Canadian side of the border. The buyer will have to supply proof of purchase and pay tax at the border. Make sure they have the receipt for the money order or bank draft and a copy of the add [if any] that was posted for the sale of the car. Apparently the border people have been watching too much Barrett-Jackson and think any 1960's era car is worth 15 times more than it actually is. I had the bill of sale, receipt, title, registration original black plate and export permit but was still three hours at the border listening to the agents tell me the amount of time I would spend in jail for trying to defraud the government. I didn't bring the receipt for the bank draft with me.
They finally let me go when I told them if they think that clapped out Firebird is worth $35,00 give me the money and you can have the car.
The pleasures of living in a socialist police state.