I'm thinking of refusing to dress my son in a bright pink onesie my mother-in-law bought and selling it on eBay
Why do you want to get rid of it?
It is for a girl.
It is not good to resell a gift, so stop focusing on the fact that it is pink or that you don't like dressing him up. It will upset people to know that you are giving away gifts for your son.
Pink is such a girly color. I don't feel comfortable with dressing my son up like that.
A color is just a color, don't get hung up on it. You don't have to dress your son in the onesie, but don't sell gifts online.