Mark Cuban -- I Busted My Billionaire Ass ... To Sell Mavs Tix (VIDEO)

Don't ever question Mark Cuban's work ethic -- 'cause the guy was so determined to make the Mavs a success after he bought the team, he PERSONALLY cold called hundreds of people to sell tickets.

It's a pretty impressive story ... Cuban says he's all about getting in the trenches and doing the work himself if duty calls -- and back when the Mavs sucked, duty definitely called.

One other thing ... Cuban says he respects the hell out of telemarketers -- and instead of hanging up on 'em (like most people), he listens to the pitch and offers feedback!!!