Argos seem to take anything back except things that they can't for hygiene purposes, which is generally exempt from legislation. An mp3 player has headphones which go in your ear, so much like jewelery it need not be taken back. Same goes for underwear in clothing shops.
You still have your statutory rights in case of defect etc, but you can't simply change your mind (which you can with most items in Argos).
You mean those hard plastic bubble wrapped yokes, that are often stapled together as well, often wondered about that, but thinking about it.... why not take it back, how are we supposed to see if we like it (without) taking it out. Lets know how ya get on.
Argos have a view policy so you can go to their Customer Service Desk and ask to view any item before deciding on purchase. If they are very busy they sometimes don't entertain these type of views.
Point is that they open any packaging to let potential purchasers view the item so who's to say that your item wasn't like this when you bought it. I've bought items in the past where they were resealed presumably before being returned or by staff.