You can sell a FooPet back to the Adoption Center at any time. Go to your profile tab, and click on the pet you want to sell. Then click "Sell Pet". The price of the pet is autogenerated based on your pet's stats. You cannot sell your pet to another member.

You cannot sell your last FooPet - you must own at least one pet. Remember that Baby FooPets must be weaned before you can sell them.