The mic Cardi hurled into the crowd at Drai's Nightclub actually belongs to an audio production company called The Wave in Sin City, and the owner tells us he decided to auction it off to benefit a couple of charities.

Scott Fisher tells TMZ … it was pretty easy to track down the mic after the show, because the white tape across the bottom of it says "MAIN," and it still works despite Cardi chucking it into the crowd. Scott says his company provides audio support for most of the major nightclubs in Vegas, including Drai's.

Videos of Cardi B throwing the mic have gone viral ... and as we first reported, she's now listed as a suspect for battery because a woman who says she was struck by the mic went to police and reported the incident.

As you probably know by now ... Cardi B was performing during a scorching hot day, and had asked the crowd to douse her with water to cool her off, but later she lost her cool when someone threw a cup full of liquid and ice at her, retaliating with the mic toss.

The starting bid on the eBay listing is $500 and Scott says the mic originally cost $1,000 ... so it will be interesting to see where the winning bid lands.