I need to buy a gift, do you think I could get some money selling my cousin's game system?
What? Why would you sell your cousin's game system?
Because I really want the gift and my cousin doesn't need the game system.
Selling something that belongs to your cousin just because you want to buy yourself a gift is wrong. You can't take things that don't belong to you and sell them, it's highly unethical.
I'm using the money to buy myself a gift that I want, I'm not a thief.
I think your cousin would disagree. Maybe you should just do some odd jobs for some extra money.