I came across the term in my early 20's when I mentioned to a friend who was in his 40's that I had gone to Rawhide, a leather bar in New Orleans.
He yelled at me and said, "Oh, you're really out of control aren't you!! First you tell me you are out there tricking with guys every weekend, (I had slept with two guys in a month) and now you are going for the "rough trade," he told me.
Ok so he was a bit dramatic, but I burst out laughing at this "rough trade" term that I had never heard of, and I asked him to explain WTF he was talking about.
He said that trade referred to basically the meaning of the word--that you are willing to "exchange"-- but the exchange centering on sexual acts. This for that. Basically, subverting yourself in some way in return for sexual gratification, money, or maybe even trading your boundaries to satisfy your desperation. Sex was the common denominator, no matter who was the beneficiary of whatever.
Let's not forget that this term was around when homosexuality was still thought to be a mental illness by the APA. So needless to say, stigma probably created some desperation where niceties were overlooked in favor of sex acts that probably were traded among men, as opposed to some organic dating or romantic notion of guys hooking up today. (Cut to the chase, you let me fuck you and I will suck your dick, deal?)
I guess over time, you just referred to both people and sexual acts as "trade" because it boiled down to a willing participant in an exchange, and not necessarily a selfless act or even a friendly or romantic interlude, just "this for that"
If you want to get asort of feel about it rent the movie "Cruising" done in 1980 with Al Pacino. Although it was a serial killer that was trying to get victims that were vulnerable because they were looking for "trade" and probably over dramatized, you might get a sense of what trade was like. Things have changed!