Prostitution
From Memory Alpha, the free Star Trek reference
Prostitution is the selling of sexual favors for money or other things of value.
Q revealed that if Quinn never inspired Sir Isaac Newton, Newton would have ended up in a prison as a suspect in several prostitute murders. (VOY: "Death Wish")
The malevolent entity, Redjac, murdered numerous prostitutes in London from 1888-1891 during his time on Earth as Jack the Ripper. (TOS: "Wolf in the Fold")
In 2004 on Earth, a man named Loomis was kidnapping various people in Detroit, Michigan for their blood types. Among these people was a prostitute, who had donated blood earlier so that they would check for sexually transmitted diseases. (ENT: "Carpenter Street")
While stranded in San Francisco of 1893, Data was offered the contact of Lillian, a prostitute, by the bellboy of Hotel Brian, Jack London. (TNG: "Time's Arrow")
Prostitution still existed on Farius Prime. In 2374, Bilby arranged for a Farian prostitute for Connelly, but he claimed to have already met a woman who "worked at a flower shop", paid the woman and had her leave. (DS9: "Honor Among Thieves")
Unofficially, Quark's on Deep Space 9 featured prostitution, in the form of holographic "entertainment." (DS9: "Meridian", "Profit and Lace")
In 2370, Kira Nerys posed as a prostitute to get past the security guards at the Hutet labor camp. (DS9: "The Homecoming")
In 2372 Captain Kathryn Janeway posed as a prostitute in order to gain access to the prison cell, where B'Elanna Torres and Tuvok were held captive by the Mokra Order. (VOY: "Resistance")
