It's only a "private transaction" if you aren't doing it for the intention of making a profit. (depending on exact laws). Unless the laws in their state are so

It's only a "private transaction" if you aren't doing it for the intention of making a profit. (depending on exact laws). Unless the laws in their state are solely based on volume, they are almost certainly acting as a dealer, which requires a dealer license and a bunch more legal requirements. In most states once someone buys a car with the intention of reselling it rather than using it, they automatically become a dealer.