I'm going to sell a gift that I was given and that I don't like.
Why would you sell a gift that someone else gave you?
I don't want it and I would rather have the cash from selling it.
I know that you probably don't like the gift, but someone gave that to you because they thought you would like it. Selling the gift shows the person who gave it to you that you don't appreciate it. It's not proper to sell the gift.
I need the cash, though, so I'm going to sell it to the gift shop.
I can understand that if you are in a precarious financial position, then gifts can seem frivolous.  As long as you don't act too bluntly in telling the giver that you did not appreciate the gift, then they will probably not be offended.  It might be best, in future, to let people know that you would prefer cash.