PLEASE HELP!!!?

HI, I've ordered a portable air conditioner on ebay (seller is curry business) . It was delivered after a week and soon developed a fault so I contacted the seller and was told to go through the ebay website to commence the return process, which I did.

The Ebay website returned label was rejected by the post office because the item weighed 25.5kg but label contained 2kg. I was told by the post office to pay for the return of the item. It is the responsibility of the seller to pick it up but they pushed me around on the phone for 5 days fruitlessly. So I paid about £40 for retured postage.  Item cost £229.00.

Parcel company has returned item and it was signed for according to its website. 

Well, over a month and half now I haven't been refunded.  Curry business keeps giving me the push around on the phone and they've ignored my emails once I mentioned "refund".

Ebay is curtently avoiding telephone customers support since the pandemic. Ebay website's  customer support is a rubbish which doesn't reflect my situation, instead the website is telling me that I have not returned item.

I'm anticipating a court action against seller or Ebay to recover both postage and item cost. Please please, what do you think? Should I sue only the seller? The seller is not an individual but a well established business institution. That's  Curry PC world online store. 

Thank you in advance