I am surprised that you could not use Parcelforce to send the part - they do take all car parts apart from a few exclusions (panels, doors, bumpers and bonnets). I know from working at a Post Office that Parcelforce accept exhausts for example.
If you had one of those excluded parts to return and could not use Parcelforce, you needed to contact Ebay Customer services to ask them to get the seller to send you a label you could use.
As soon as you open a case, the clock is ticking, and you only have about 10 days to get the item sent back. I don't think Ebay will get involved now as you didn't get the item sent back in time and the case is closed.
Usually, if you receive something that is not what you ordered, or the item is defective, you may be able to get a refund from your payment provider.
However, without being harsh, it seems that you did not do your research thoroughly and ordered the wrong part.
I think your choices now are limited.
You could arrange the courier yourself - if you book and pay, then the contract is with you and you can pursue it if they don't turn up.
Unless it is a large part and on the Parcelforce excluded list, you could return it using Parcelforce.
You could sell it on yourself as a collection only item to recoup at least some of your money.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)