My understanding of the situation is similar to what argenvans says. A local argentinian cannot buy a foreign registered used car here in Argentina. The only way for a local to buy a used car from the US in the country is 1) to be a diplomat, or 2) live abroad for 1 year and bring the car in as a personal item (this can only be done once.) "Taxes" are not what prohibits locals from buying foreign used cars. A local argentinian can, however, import a new car and pay substantial taxes in the 100% range. At least that is my understanding.
Also, a foreigner here should in theory have no problem buying the car. As Sleuth said, they need to get the proper paperwork sent from the DMV in the US to Argentina. The problem is the foreigner will only be able to keep the car in the country for the duration of their respective visa. So, those of us here on 3 month tourist visas would have to drive the car to Uruguay every three months. On top of this, no argentinian would be allowed to drive it.
If this info is not correct, please let me know, cause I have been wanting to either buy a car from a foreigner or bring my car in from the US. I am sick of driving my crappy (and wildly over-priced) vw golf.