If you send a customer the wrong item or a defective or counterfeit item, you are obliged to provide an exchange or refund.
Other than that, it's your shop, you can set the policies to be whatever you like. Bear in mind you are selling online where customers can't see or touch the items before they order them, so you are more likely to get sales if customers know they can return the items if they don't like them.
Please be sure to list all your policies in the policies section of your shop profile!
Try to do the right thing.