For those of you who want to know, here is the basic process for selling a US plated car to a foreigner in South America.
This is all that needs to happen for the old owner to no longer be legally responsible for the vehicle and for the new owner to drive it legally anywhere in the world.
There are a few more details on Wikioverland: http://wikioverland.org/Argentina#Buy_or_sell_a_car_or_motorbike_in_Argentina
OPTIONAL: The new owner can choose to re-register the car in their name in the United States. There are two ways to do this. Note that this is NOT required in order to drive the car in Central or South America.
OPTION A: Drive the car to California, hand over the signed title, and register the car. Because California has emissions testing you must present yourself and the vehicle in person. They will transfer the Washington State title to a California title and you get to pay happy California registration fees. For more information on this visit: http://www.dmv.ca.gov/vr/vr.htm
Note that you can register this car in any state that you want. California is the closest stop, and a very easy place for foreigners to register cars. But you could legally register in any state. (And in any country for that matter, assuming you are willing to pay the import taxes). Some states (like Washington) will charge a late fee if you register the car more than 15 days after it was reported sold. In Washington it's $2 per day to maximum of $125. It's not a big deal, but you should know.
OPTION B: The buyer can apply for a new Washington State title by mail. (Note that we are still verifying the details of this process and we are not yet certain this would work for anyone but Washington State residents).
That's all I know. We will update the last section as we recieve more information from the DOL.
The seller magically finds a buyer. (They can be from anywhere, except Argentina.)
The seller creates a bill of sale for both parties.
The seller signs the title and hands it over to the buyer.
The seller and buyer go to a notary public (i.e. Lawyer) in Buenos Aires and have a power of attorney created. This notarized paper states simply that the seller has permission to drive the car everywhere for an unlimited period of time.
Seller and buyer go to a customs office in Buenos Aires (or cross a border). Here the temporary vechicle permit is transferred from the old owner to the new owner. This allows the old owner to leave the country. And gives the new owner the correct paperwork to legally be driving in Argentina.
The seller reports the car sold to the Department of Licensing in Seattle. This removes all responsibility from the seller for things like traffic tickets incurred.