You don't pay your tax to Ebay, but to HMRC.
When you complete your self assessment, you can deduct legitimate expenses, such as postage costs. You just need to do a bit more work.
"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)