Think it's okay if I'm thinking of selling my son's shoes?
Why do you want to sell the shoes?
I want to sell them to my girlfriend's mom
It does not seem very moral to take your son's shoes and sell them. These are your son's, and they will probably be upset that you did this without asking them.
I'm doing him a favor, those shoes are awful.
But what you are doing is still considered stealing. You shouldn't be selling anything that isn't yours.