Company Inscriptagraphs tells us ... they've been hit with an increase in web traffic and sales hours after O.J.'s death was confirmed by his family. We're told the owner even opened the store early due to the high demand.

40 sales were made the morning after O.J.'s death from items ranging from $150-$2,000 in pricing -- and Inscriptagraphs is planning to add more inventory to its already impressive collections. The business is located in Tivoli Village ... minutes away from where O.J. lived in Sin City.

The store boasts a large collection of O.J. memorabilia, including a signed Buffalo Bills jersey, autographed baseballs, action figures, as well as the late athlete's Rookie trading card -- which is also signed. Inscriptagraphs also has in its possession a signed photo that was confiscated as evidence in O.J.'s 2007 robbery trial.

According to store staffers, O.J. has always been a top seller for them ... claiming people love having his memorabilia as conversation pieces. They have a large inventory of O.J. memorabilia since they have had access through the years being Vegas-based.

Remember, the former NFL star became an infamous American figure after he was put on trial for the 1994 murders of Nicole Brown Simpson and Ron Goldman. O.J. was acquitted in the highly publicized trial ... but later found liable for their deaths in a 1997 civil trial.