You have the ability to change my character's gender. I have money. Let's make an exchange!
But seriously, this is a very common cosmetic service available in many MMOs. I would be happy to pay anywhere from $10 - 20 for it. I made a female human Hunter and 50 levels later regret not making a male, but there's simply no way I'm going to start over now. Please consider adding this essentially industry standard service to the store. It has absolutely no negative impact on the game while potentially being a nice source of income for you.