I reckon they'll sell Benzema to Arsenal if they acquired Aguero. Funny enough while @Akiva is right the Aguero is a far better striker than Benzema, Benzema is the better team footballer. His movement, control, quick one touch passing plays a unique role in their offense. For that reason, I really don't see Madrid being interested in Aguero. But if they were and purchased Aguero moving Benzema would not be that hard. Madrid moves players they don't want easily all the time.