How it works (I think) is that most expensive cars in the UCD have a 'base price'. And the actual price of the car in the UCD goes down depending on how many miles it has. So you could by car with alot of miles that costed you 967,XXX credits. While that car cost you less than 1,000,000 credits, the base price is 1,000,000 credits, therefore making it untradeable. The only time you'll ever buy standard cars at their base price is when they are in the OCD with 0 miles.
