In the western world, women want rights, independence, and freedom. If this was legalized that would be highly conflicting. I understand where you are coming from, but from a legal standpoint it would be awful to deal with. There would be higher rate of rape, accused rape, etc. Overall would be too much trouble if you ask me.
And what about guys. If women can sell themselves, then does that mean there can be a market where women can buy males? To say that men are always in urgency to have sex instead of women is subjective.