"a promotional advertisement," 1958 (in Billboard magazine headlines), shortening of promotion in the sense "advertising, publicity."
"graft" (especially to disc jockeys from record companies to play their music), 1938 (in a Variety magazine headline), from pay off "bribery" (underworld slang from 1930) + ending from Victrola, etc. (see Pianola). Compare also plugola "surreptitious promotion of a person or product for a bribe" (1959), from plug (n.) in the advertising sense.
1838, "a hoax or planned deception by which a victim is 'taken in,' " from sell (v.). The sense of "advertising technique" is attested by 1952 in the phrase hard sell.