First, have you contacted the seller and offered to help him/her file a claim with the carrier? And asked for a refund (and a return label if s/he wants the box back)?
Go to your Purchase History for that item, click the link for more options and select that you want to return the item, choosing the option that what you received was not as the listing described it (if you get a text box to add information mention the apparent tampering), then follow instructions from there. If you haven't gotten a label to return the box or a full refund after 3 days (but before 21 days) from doing so go back and you should find a link to "Ask eBay to step in" to get your refund.
