Both of these words are Cant (that is to say, codewords used to disguise one's shady business). "John", like most cant words, was probably chosen specifically to be a code and agreed upon by most in the business, thusly passed on to next generations of prostitutes. Also, like most cant words, John probably has no tracible origins. Trick, however, while still hard to trace, has a few possibilities. Technically, the obscurity of the word has led it to yeild many translations, including: Prostitute, the act of prostitution, and a patron to a prostitute. The latter being (as far as I know it) the most correct one. As follows are possibilities: 1) Meretrix, the latin word for prostitute. Although you could really shave it down to just "trix" (the latin suffix for a female) it's possible someone happened upon this word in particular and rebirthed it. Especially if said person was looking for a cant-word and chose latin? It could happen. 2) The expression "Turned the trick", while better known as "(that) did the trick" [meaning- got the job done], could be linked in some way to the modern use. The theory is given cogency in that the act of prostitution is commonly known as "turning tricks". 3) Trichomoniasis, an STD common in promiscuious women (like those engaged in prostitution). Often shortened to "trick". How this became linked to prostitutes themselves is a tough one, but I figured I'd give it as a possibility. I'd also mention that another slang refers to anyone who is in a relationship for the other person's money is often called a "trick" (gold-digger, where I come from). It's possible that the slang in question was birthed from another slang... or visa versa. Hope this helps.

Can't say about trick, but I've alway's imagined that "John" was simply the most commonly used phony name. "So, what's your name?" "Uh, John."