I'm thinking of selling something that isn't mine.
How are you going to sell something that isn't yours?
It was supposed to be someone else's gift for a friend for their birthday, but I took it.
It's wrong to steal a gift from your friend. You should return it back before the gifter wonders where it went.
There's no proof that I stole it so it's fine.
Even if there's no proof that you took you should still give it back. Stealing is wrong, and stealing from a friend is even worse.