monopolist (n.)

"one who has exclusive command or control of some branch of trade or article of commerce," c. 1600; see monopoly + -ist.

The concept is older than the word. Middle English had regrater "monopolist, one who buys up goods before they come to market" (late 14c.; early 13c. as a surname), from Old French regrateor and Medieval Latin regrator. There also was a verb, regrate, regrating.