I've returned an LE before without any problems. If there's no mention of it on the receipt or on the item your purchasing, then they have to take it back or at the very least, give you credit. I would make sure to get the employee name and then ask to speak to their manager. If there's still a problem, ask to speak to an area or department manager. Basically, keep complaining until they give you your money back. No one at Disney wants an irate customer in their store!! Also, the refund/return policy should always be clearly stated or posted and if it's not, the ball is in your court and at your advantage.
I'm sure Disney pin shopping locations get flooded with people buying the maximum 2 pins per person with the intention of trading or selling the pins. Then when the pin turns out to be a complete dud and no one wants it, these people return the pins which is probably super annoying to employees and managers. BUT, if they want to release LE pins, this is what they're going to have to deal with. Some pins are hot, others are not.
I once saw someone return a sold-out LE pin and the next person in line is like..."I'll buy that pin that they just returned." lol