For USFK/SOFA personnel, 19th Theater Support Command has an informative site regarding the purchase of a used car in Korea. Click on "Newcomer," then "Settling/Processing In" and scroll down to the relevant info. 
http://www-19thtaacom.korea.army.mil
For those wishing to sell their cars:
Please be advised that due to tax reasons and other factors such as high registration and inspection costs, we generally are only able to purchase SOFA-plated vehicles originally purchased on the local economy. 

The exception we sometimes make is for certain US/European vehicles 5 years old or older, in which case the above costs (primarily the official inspection which currently stands at about 4-5 million won along with a 6-7 month waiting period) are subtracted from our offer, which can result in a figure significantly lower than the seller may expect.  
                  ~Drive Safely~
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