You can report the seller to Ebay for abusing the Money Back Guarantee and then ask Ebay if they will perhaps refund the postage costs to you.
Generally, if buyers have to open a case for each return, they do need to send each return using a separate label so they have evidence that the seller has received the returns.
"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)