The son of Tapit was retired with a perfect record less than 24 hours after his emphatic triumph on the Kentucky dirt on Saturday.
Trained during his unbeaten six-race career by John Sadler, he will stand at Lane's End Farm with a fee yet to be decided.
With Flightline fever still ringing around Lexington, Lane's End - who are one of the world's premier thoroughbred farms - and Keeneland joined forces to offer a 2.5 per cent fractional share in what is sure to be one of racing's hottest stallion prospects.
Seitz faced plenty of opposition during the auction, with Coolmore the eventual underbidders.
Flightline is by Tapit and out of the Indian Charlie mare, Feathered. He was bought by West Point Thoroughbreds for $1m in 2019 and raced in a partnership which also included Hronis Racing, Siena Farm, breeder Jane Lyon's Summer Wind Equine and Woodford Racing.
"He was really excited about this horse and just wanted to make a big splash. He's dealt with some nice horses in the past but obviously nothing like this horse - I'm not sure any of us have," agent Fred Seitz told Catena Media.
"I'd like to be able to tell you what his name is but right now he'd like to stay confidential. Maybe at a later date he'll tell me to reveal it but right now he asked me not to say who it was."