As difficult as this may be to believe today, when one's sexual orientation is typically defined based on the physiological gender of one's sexual partner, for the vast majority of human history up until about the 1950s, in nearly all cultures and societies, sexual orientation was defined based not on the gender of one's partner but on the type of role one played in the sexual encounter.
In 19th century and early 20th century London and New York, the word trade was a slang term used in male homosexual communities to describe a man who normally slept with women, but was willing to sleep with other men as long as he was the "top" in the encounter. The term "trade" derived from the fact that in most cases the man would be paid for his services with money or other remuneration.
What is so interesting about this concept is that while the man who accepted penetration by other men was seen as abnormal and discriminated against during this period, the "trade", because he only ever played a traditionally masculine role in the sexual encounter, was in no way looked down upon, or considered abnormal in any way. Indeed, he might even be specifically sought out by his wealthier partner for his perceived hypermasculinity, and might even be asked to engage in sadistic behavior toward the "bottom," in a practice referred to euphemistically as "rough trade." Because of these two aspects of monetary exchange and a desire for masculine men in the pursuing of sex with "trade," the typical image of a "trade" in 19th century New York was of a relatively poor but burly manual laborer, such as a dockworker.
The reason why "trade" as a slang term in the homosexual world is almost unknown today is that since the 1950s, shifting conceptions of homosexuality and heterosexuality have led to the disappearance of "trade." As homosexuality increasingly came to be defined by one's choice of partner rather than one's sexual role, the number of "heterosexual" men willing to sleep with other men on occasion for money drastically declined, to basically zero, as these men could no longer engage in this practice without risking severe stigmatization. It was in this context that when older homosexuals were interviewed by researchers in the 1970s and 1980s, many of them claimed that life was better for them in the early half of the 20th century and bitterly complained that nowadays there were no more "trade" to sleep with.