Or as Blizzard calls it, Re-Customization:
Character Re-Customization is a paid service that lets you change an existing character's gender, face, skin color, and other cosmetic features determined by his or her race and gender combination. When you perform a Character Re-Customization, you may also change the character's hair color and hair style (similar to the in-game barbershop) and select a new name, if desired.
I dunno, I don't think I would pay $15 to change my character's gender. But then again, it's better than leveling a brand new character. I know some people who really regretted making their character a specific gender, and I guess they would be happy with this.
Does this count as RMT? It sort of is, and it sort of isn't. To be honest, more than anything else, I think the price for this, name changes, and server transfers is to offer the service to players who really want it, but keep the demand low.