Limit on the number of returns?
Speaking from a retail background; many stores which allow returns or exchanges without a receipt will do so in good faith for the customer. Someone who has been refused for a return will typically be for a reason. ie. An associate saw the customer pick the item up off a shelf and then walk to the counter to make a refund with an old receipt. Or someone who is in the store often but never seems to buy something yet has items to return often and claims the purchase was at another location, or a gift.
An employee working in a good environment, for a company which makes them happy will watch out for the company and will feel a sense of guilt if they allow a customer to make a return when the situation makes the associate uncomfortable knowing or 'feeling' the product was not attained honestly.