Green’s Dictionary of Slang

trade n.

1. [late 16C+] (also trading) prostitution, sexual intercourse.

2. [1920s+] a prostitute’s client.

3. [1910s+] (gay) a man with whom one has (commercial) sex, a male prostitute; a male prostitute’s customer; thus piece of trade under piece n.

4. [1940s] one who works as a prostitute.

In phrases

take it out in trade (v.)

[1940s+] (Can./US) to have sexual intercourse as the ‘price’ of taking a woman out.