Monkey Business

The New York Times Magazine reports that when capuchin monkeys in a Yale experiment were taught to use "money" (metal tokens they could exchange for food), they began to trade the money for sex.

What [the researcher] witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: they monkey who was paid for sex immediately traded the token in for a grape.)
-- The New York Times Magazine, June 5, 2005

Maybe it's the "oldest profession" because it's the most obvious.