You shouldn't be able to sell premium items

They should make it so the only things that can be sold are the current in-game cash items.

Any premium or limited time cash items are blocked from being sold

Another thing they can do is add a password feature. Where you're required to enter a 4 digit passcode whenever something is either being sold or purchased. Just for added security.

That way if a person enters your town by hack or glitch, the most they can do is store your stuff or re-decorate your town