Green’s Dictionary of Slang

(US drugs) a small- or medium-time drug dealer.

1969		Smith & Gay Heroin in Perspective (1972) 105: The customer of the kilo connection [...] may also be called an ounce man.
1989		(con. 1950s) Courtwright & Des Jarlais Addicts Who Survived 202: What we used to call the ‘ounce men’: people who buy a quarter kilo, mix it a couple of times, and sell it in ounces.
1995		G. Cuomo Couple of Cops 162: But we quickly realized that they were what we called ounce dealers, with a batch of coke on the kitchen table that they’d been shaving down with a razor and packaging to sell to their street dealers.