Brand new autographed O.J. Simpson memorabilia has already hit the market -- just 10 days after Juice signed a bunch of helmets in a secret Vegas hotel signing session ... TMZ Sports has learned.

Steiner Sports -- one of the biggest memorabilia dealers in the business -- just posted two new signed Buffalo Bills helmets in the "new arrivals" section of their website. One is selling for $400. The other, containing O.J.'s Hall of Fame year, is going for $450.

The website claims the helmets have been "hand signed" by Simpson and the autograph has been deemed "100% authentic."

A rep for Steiner tells us, "So it is clear, Steiner Sports did not hold the signing with O.J. Simpson, but purchased his new autographs from a third party."

Steiner CEO Brandon Steiner tells TMZ Sports, "Collectors collect and O.J. Simpson is a part of sports history for his accomplishments on the field. He’s a Heisman Trophy winner, 10,000+ yard rusher and Hall of Famer. We are just trying to answer the demand of our customers looking to complete their collections."

The big question ... with O.J. merchandise selling in the hundreds of dollars, you gotta wonder if Simpson is being paid for his Hancock.

If he is, O.J. owes all of it to the Goldman family -- thanks to the wrongful death judgment against him. If O.J. tries to hide the money, he could go back to prison.