TMZ Sports has learned ... the stars and stripes that Brady signed and then re-signed following a man's dispute with New England's HOF in 2021 is going up for sale on eBay later this month -- and its starting price tag is $299,999.

Unclear what it's ultimately expected to sell for ... but we're told the profits will go to a good cause nonetheless.

Dan Vitale -- the owner of the flag who feuded with Patriots officials after he alleged they ruined the item after putting it on display at the team's HOF -- is donating a portion of the proceeds to the Behavioral Health and Developmental Services of Strafford County, Inc. -- a non-profit whose goal is to help those with autism. Vitale says the rest of the funds will go into a trust for his daughter, who has autism.

When it eventually sells, the new owner of the flag will be getting a sweet piece of Brady memorabilia -- one that has quite the story attached to it too.

As we reported, it initially flew over Foxboro Stadium during a New England vs Miami game in '01 -- the same year Brady first took over as Patriots starter. Brady then signed it years later.

Vitale ultimately bought it and handed it off to the Pats Hall of Fame to display it -- but he claims officials there caused the signature to fade when they improperly took care of it. He sued in 2022, alleging their mistakes decreased the value of the piece by $1 million.

Brady, though, came to the rescue ... re-signing it in big, black ink following the Buccaneers vs. Seahawks game last year. Vitale said he was stoked with the new signature -- and believed it was worth over $1 mil again.

“While the flag holds a lot of value to me personally," Vitale said, "I could not be more thrilled to auction it on eBay so another collector can own this rare piece of sports history and in the process, support those with autism -- including my youngest daughter.”

Bidding on the item begins on March 23 ... and will end on April 1.

Good luck!!!