I wouldn't be at all surprised if the USAF had bought them for evaluation purposes.
If some civilian millionare/billionare had bought them there would no-doubt be a bit of information about it...and even some youtube videos from the proud new owner.
Still...$5mil isn't much to pay for a (recently) front-line fighter. I might have to start buying lottery tickets, I could see myself zipping around in one of those!! Is it a 1-off, or could Proud repeat the effort for a wannabe buyer (who just won the lottery)?