The absolute key thing here is the tracking.
I assume you returned the trainers using a tracked method, through an Ebay case, to the address the seller provided?
If you did all of those things, and the tracking shows that there was an attempt to deliver to the seller, then you can appeal.
If you didn't open a case, or send using a tracked method, then you could perhaps cut your losses and sell the trainers yourself.
"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)