Tau was the a thief who sold weapons and technology that his ships confiscated from other passing starships. His business made him a very rich man, and put him in control of "the seventh province in the north" on a planet located in the Delta Quadrant. He dealt mainly in weapons, and if a buyer balked at his price, he would threaten to sell the weapons to the buyer's enemies.
In 2374, he stole USS Voyager's main computer processor, a holographic mobile emitter, and other items right through Voyager's shielding using a stolen translocator device. In addition to these physical items, he acquired from Voyager, Captain Kathryn Janeway's Leonardo da Vinci hologram. Tau became Leonardo's "patron" and sold his inventions to the highest bidder.
Janeway eventually discovered Tau's location, rescued Leonardo, and recovered Voyager's stolen property. (VOY: "Concerning Flight")