Trying to refund a Xbox One that hasn't been delivered, but i can't because it won't let me.

[dillcatr]

So i bought an Xbox One on Ebay from a seller for £200, a day later i realise i have a £80 Argos giftcard which i can use to get a brand new Xbox One instead for around the same price as the one from Ebay. The item has a 14 day refund so i contact the seller and i asked if i can return it, he replied saying that if i send it back to him i can get a refund. So i agree to this and i was going to send it as soon as i recieved it in the mail. 2 days after the delivery date i still haven't recieved it, i decide to get refund because it hasn't been delivered. So i cancel the return case and filled in the reason as 'I want a refund'. I go to open a refund case but it won't let me. I do a quick google search and apparently you're only allowed to have one case per item. What do i do now?! I haven't got the item and it won't let me do anything about it. I'm trying to contact the seller but he hasn't replied since.