A maraji crystal was a controlled addictive substance illegal in the Cardassian Union. According to Quark, Cardassians didn't like the crystals (or possibly their effects) and Bajorans were too poor to buy them.
In 2366, a spy for the Romulans, named Livara, was reportedly peddling maraji crystals on Terok Nor. Odo was investigating Livara's actions, and discovered that Livara had tried to sell the crystals to Quark.
In a hallucination experienced by Deep Space 9 personnel in 2373, a distorted view of this event was presented, where the inquiry about Livara's maraji smuggling was made by Thrax, rather than Odo. (DS9: "Things Past")