It sells, in an axiom

SEX

This three-letter word is said to move products better than any advertisement.

This clue plays on the phrase "sex sells," a well-known marketing axiom suggesting that using sex appeal can boost sales. The word "axiom" hints at a common saying or truism, while "it sells" directly points to the last word in that phrase. By dropping "sells" into the axiom, you're left with "sex" as the answer.