I'm going to change the locks without telling my ex
This sounds like a serious situation. Why would you change the locks?
She promised she wouldn't sell my stuff until I give her the new key
I get your concern, but you shouldn't be living with your ex in the first place. Just tell her the truth about doing it.
Nope, I'm going to change the locks and make her pay for the new ones.
If you weren't living with your ex these complications and temptations to do mischief would not happen. It is best to live away from your ex.