Without more context I cannot say with a 100% certainty, but (1) this is not a common expression, and (2) it probably means that someone has a keen sense for what is marketable. English, as a language, likes and indeeds welcomes original expressions, particularly if they are clever in some way. "Smelling what's selling" rhymes and connotes that someone has a nose/good sense for what products will be successful on the market.