As we recently reported, Brit's former Kentwood home is on the market for $1.2M after the star's estranged father, Jamie Spears, sold it two and half years ago for a mere $289K.

Much of the 3-bed, 3-bathroom home's OG furnishings have been preserved -- including a very damning door statement, which tells us that Britney's rivalry with Xtina has always been "Toxic."

"Christina sucks, Brit rules" is scribbled in black marker across a wooden door ... which, if put up for auction, could see fans dropping HUGE sums of money to get their hands on the rare, one-of-a-kind nineties memorabilia.

Bobby Livingston, executive VP at RR Auction, tells TMZ that the door could bag anywhere between $20-$30K.

Jared Mast, head of pop culture at Goldin Auctions, says that the handwriting would first need to be authenticated in order to know the true value. If that message is in Britney's handwriting, he could see the door selling for between $10,000-$20,000 given the background.

Ed Kosinski of Gotta Have Rock and Roll reckons it could go between $20-$25K ... while Darren Julien, CEO of Julien's Auctions, speculates to us it would go for only between $4-6K.