I love always such questions, when you wanna know the terminologies and origin, and histories as well as mechanism that involve coined expressions etc. in English language. coz simply it is my hoby to dig those funs and I do recollect them. So let me take you to whole answer splitting them as follows.

One that acts as an agent for others, as in negotiating contracts, purchases, or sales in return for a fee or commission.
A stockbroker.
A power broker.
TRANSITIVE VERB:
bro·kered , bro·ker·ing , bro·kers
To arrange or manage as a broker: broker an agreement among opposing factions.

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ETYMOLOGY:
Middle English, from Anglo-Norman brocour, abrocour; ; akin to Spanish alboroque, ceremonial gift at conclusion of business deal, from Arabic al-barka, the blessing, colloquial variant of al-baraka : al-, the + baraka, blessing, divine favor (from braka, to bless; see brk in Semitic roots)

WORD HISTORY:
Giving gifts to one's broker might be justifiable from an etymological point of view because the word broker may be connected through its Anglo-Norman source, brocour, abrocour, with Spanish alboroque, meaning "ceremony or ceremonial gift after the conclusion of a business deal." If this connection does exist, "business deal" is the notion shared by the Spanish and Anglo-Norman words because brocour referred to the middleman in transactions. The English word broker is first found in Middle English in 1355, several centuries before we find instances of its familiar compounds pawnbroker, first recorded in 1687, and stockbroker, first recorded in 1706.
Hesitate not to ask me again if anything spring up in your mind like that funny questions. Best of luck.