Farrah Abraham -- Used Sex Tape Bikini For Sale

Rocky's boxing gloves ... Dorothy's ruby slippers ... Indiana Jones' bullwhip ... the bikini Farrah Abraham wore on the cover of her sex tape -- one of these pieces of cinema history CAN BE YOURS!!! --- take a guess which.

Vivid Entertainment -- the porn studio behind the release of "Farrah Superstar: Backdoor Teen Mom" -- listed Farrah's pink crystal-encrusted two-piece on eBay at midnight ... and bids started at $500.

It's pretty hilarious ... the listing advertises the Poolside Collection bikini as "used" -- we're told that means exactly what you think it means: the bikini wasn't washed since Farrah wore it. Think Monica Lewinsky and The Gap.

In fact, we're told the bikini shows overt signs of wear, including fake tan stains and glitter from Farrah's body lotion.

As the listing goes ... "Farrah's bikini ... straight off Farrah's body and into your hands."

As if your hands won't already be busy enough.