Seller only gave partial refund even though item was returned exactly as received

[pjee9938]

I only received half of a refund even though I returned the item new. The Seller claims I opened the packaging, but I did not. The whole reason I returned the item in the first place was because I was tired of waiting for the Seller to ship and I found the item on amazon for cheaper, and received it sooner than the Seller could even begin the shipping process.  I bought the replacement on 4/23 and notified the Seller (who had not shipped yet) to simply cancel the order. Something they should have been able to do for me easily. But instead they refused to cooperate and shipped the item anyway on 4/29, the last day before I could make a claim to Ebay to cancel the sale. Now, I received the replacement item from Amazon on 4/26 and was satisfied with that. I received the Ebay Sellers item on 4/29. I returned the item and recently only got half of the original amount because the seller claimed that the tape on the box was tampered with and opened. I would have no reason to open up that which I already have a replacement for. I did not even remove the item from it's shipping wrap. I sent it off exactly as received, since it was already packaged to ship. Is it not possible that the Seller opened it themselves just to halve my refund? I tried to submit a case at the refund page under open a case, but every time I do, it boots me back to the refund screen. What should I do to get my full refund?