There are real estate brokers, wedding brokers, pawnbrokers, and, of course, stockbrokers.
A broker is someone who arranges or negotiates things.
It comes to English from the French wine industry, where brocour described the person who bought wine in bulk from the winery and then sold it from the tap.
The accepted meaning became anyone who bought something in order to sell it again. In English, the word brocour became broker, meaning the middleman.
Broker, in its earlier spelling, first appeared in English in 1377 in Piers Plowman:
“I haue lent lordes and ladyes my chaffare And ben her brocour after, and boughte it myself.”