Wikipedia says

"Which came first: the chicken or the egg?" The dilemma stems from the observation that all chickens hatch from eggs and all chicken eggs are laid by chickens. "Chicken-and-egg" is a metaphoric adjective describing situations where it is not clear which of two events should be considered the cause and which should be considered the effect."

Same thing with sales and marketing; which is more important? Well, it's the relationship between the two that increase your revenue.