Kim Kardashian -- NOT Sex Tape Mystery Buyer

Kim Kardashian is NOT the mystery buyer trying to yank her infamous sex tape off the market -- this according to Vivid honcho Steve Hirsch, the man who currently owns the tape.

Hirsch tells TMZ, he still doesn't know who the mystery buyer is -- but a rep for the interested party insists ... it ain't Kim ... or anyone even remotely associated with Kim.

Steve tells us, "It made sense the entire time it would be Kim, but it appears it’s not.”

According to Steve, the buyer will reveal his identity if the deal gets done -- and explain once and for all why he/she wants to pull the tape from the market.

Steve says he met with the buyer's rep several times in the last week -- but the buyer's still several million dollars short of Hirsch's $30 million asking price.