I'm going to sell my sister's panties.
Why would you sell her panties?
I want to make some money.
That is disgusting. That is a betrayal of your sister just so you can make some money. That is wrong.
It's a harmless joke, is it?
It is not harmless. It can be mortifying for a young woman to have her inmate possessions sold to creepy guys.  What do you think would happen if your parents found out?