Or if you have 388,000 Cr and don't want to wait, hover your cursor over the S7 in the Car Collection screen in the pause menu, then exit and then re-enter. It'll then give you the option to purchase it using the Y button or equivalent.
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It's probably cheaper to buy them out at the default price of 273,000 Cr but this tip can still help with some of the harder to find cars. Note that this only works with wheelspin exclusives which are the first car in their manufacturer category as the cursor jumps back to the first position when re-entering Car Collection.