And you really think it's fair on the retailer to have to return used goods? They're either going to have to sell them as used and take the hit through no fault of their own, which is hardly fair, or else sell them again as brand new, which is legally questionable. Even if they could get away with this, as I said, how would you feel if you bought a brand new item and discovered it had been opened and used by someone else?
Despite what you've been told or managed to cajole a supplier into doing, I still don't believe they are legally bound to offer a full refund on a used item simply because you decide you don't want it any more. It's just not logical or reasonable in my view. If the law does indeed permit this then it's frankly barking.