Martus Mazur was a male El-Aurian refugee from the El-Aurian system during the late-24th century. One of a handful of survivors from a Borg invasion of his planet, he used his renowned skills as a "listener" to serve his purpose as a con artist.
In mid-2370, Mazur visited Deep Space 9 where he was promptly arrested by Constable Odo for swindling a couple on the station. Martus was locked in a holding cell and met a sickly alien named Cos, who showed him a gambling device that always brought him bad luck. Cos died and Martus took the device. Martus, after being released, convinced a widow, Roana, to let him turn her shop into a gaming establishment on the station's Promenade called "Club Martus". Club Martus used large-scale replicas of the alien gambling device. At first the club was a success, and seriously damaged Quark's profits. Even Rom left Quark and became a limited partner of Martus. But soon the device began letting everyone win, cutting deep into Martus' profits. His patron Roana wanted to close him down also.
Meanwhile, an abnormal amount of unusual coincidences and minor accidents started to occur and Jadzia Dax found that the laws of probability were seriously being altered when she discovered neutrinos were not spinning as they should. The gambling devices were the cause of this and they were destroyed by phaser fire. Once again broke and under arrest, Martus was bailed out by Quark with the provision that he leave the station; one con man was enough. (DS9: "Rivals")
"But I've done nothing wrong."
"It's true. We were just talking."
"You were talking, madam. He was listening."
- - Martus Mazur, Alsia and Odo
"I know all about you El-Aurians. You're listeners. People like to talk to you."
"Everyone has to have someone to confide in, someone to hear their stories I offer a sympathetic ear. And if they request my help I give it to them."
- - Odo and Martus Mazur
"It seems overconfidence comes in small packages around here."
"My track record speaks for itself, just like your security file."
- - Martus Mazur and Quark
A long-sleeved grey and copper-colored jumpsuit with a triangular pattern and a black, white and gray jacket with gold and black trim as worn by Sarandon were later collectively sold off in the It's A Wrap! sale and auction for US$492.00. 
Martus was the first El-Aurian identified as such in dialog. Fellow El-Aurian Guinan was not revealed to be this species until Star Trek Generations. The writers intended for Martus to be Guinan's son, but the connection was dropped when they found out Whoopi Goldberg was unavailable. He was also intended to be a recurring resident on the station, a rival for Quark. However "Rivals" was not a particularly popular episode with fans and producers so the idea was dropped. (Star Trek: Deep Space Nine Companion, p. 109)
Michael Piller originally envisioned the character of Martus becoming a recurring one but later decided against it, "When we did 'Rivals' I thought that the character of Martus had a chance of being a recurring one. It was like Harry Mudd or something like that. But I don't think it will. I thought it was an average episode, albeit with some great character stuff in it." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p.67)
Armin Shimerman had previously worked with Chris Sarandon on Broadway fifteen years earlier but he didn't feel their two characters worked well together; "Chris and I got along fine, but the one-upmanship that should have been there, these two swindlers trying to outswindle each other, didn't really work." (Star Trek: Deep Space Nine Companion, p. 109)
In his review of "Rivals", Star Trek author Keith R.A. DeCandido thought Chris Sarandon, "utterly fails in the role of Mazur. He’s flat, dull, uninteresting, has none of the needed spark with Armin Shimerman that the role calls for, nor do we see any evidence of the charm he’s supposed to be using on both Alsia and Roana." 
Mazur and the events of "Rivals" are briefly referenced in the SCe eBook Sargasso Sector, in which Fabian Stevens places a bet on Julian Bashir to win the racquetball game and subsequently loses a large amount of money.
Mazur is also featured in the Star Trek Customizable Card Game.