Patrick Mahomes Holy Grail Card Collection Up For Sale For $7.5 MILLION!

A Holy Grail collection of Patrick Mahomes cards is up for sale ... and some of the pieces in the set are so rare and unique -- the asking price is a cool $7.5 MILLION!!!

The owners of the cards -- 29-year-old Robert DeArmitt Jr. and his wife, Heidi -- tell TMZ Sports the collection took about 3 years to amass ... and it all started as an investment project for the couple back in 2017.

According to Robert, he and Heidi saw Mahomes play often as Texas Tech fans ... and they just had a feeling the quarterback was going to be a beast after being drafted by the Chiefs -- so they started acquiring pieces.

Robert says one of his first big purchases was a $275 card that he bought shortly after Mahomes' first start as a rookie in K.C.

That card, Robert says, is now worth around $400K!!!

In total, Robert says he and his wife poured in around $20,000 making the collection ... and they're now looking to part ways with over 33 of the pieces for a small fortune.

Robert tells us he's already had "7-figure offers" for the set ... but he says he believes at its floor, the collection is worth AT LEAST $7.5 mil.

As for the cards in the collection ... most are 1-of-1 prints -- aka the only versions of the pieces in existence.

Almost all of the cards have some sort of Mahomes autograph or patch ... and most have perfect Gem-Mint 10 grades on them.

Robert's listed it all on eBay ... and, yeah, there is a "best offer" option.