It seems you're past any eBay Money Back Guarantee timeline on this for eBay to intervene.
It's good your seller even took it back and refunded anything.
What is the difference in cost between what you paid and what the seller refunded - is it all original shipping costs or something else? Where did it ship from to you in Cyprus and who paid the return shipping?
Unfortunately eBay won't get involved in warranty issues like this, you will need to work it out with the seller.
If he did say, and the warranty says, that they will cover shipping costs then keep pressing him about that, maybe it's a honest mistake.
He was right they couldn't refund in eBay, the order is too old to do that, so at least they took steps to refund you the cost of the part initially.
