I'll take the ATF's definition over some random internet guy.
Correctly stated, it would be "nine supreme court justices and some random internet guy".
"Straw purchase is a purchase where a purchaser knowingly acquiring an item or service for someone. The person purchase on behalf of other when the other person is unable to purchase the item or service themselves is not considered as an illegal act. But any violation of the Gun Control Act, 1968 make the straw purchase a felony. "
https://definitions.uslegal.com/s/straw-purchase/