To address the only real point you have made about your issues with Ebay, ' they took money from me because a buyer's item hadn't arrived despite me providing proof of postage ...'
You would need to provide proof of delivery - that has always been the case, otherwise your buyer is entitled to a refund.
There is no email address which deals with complaints.
The agent gave the correct advice.
However, before you make your complaint, it's worth having a read through the User Agreement.
When you start to sell on the site, you agree to all of the terms and conditions contained therein.
if you are complaining about Ebay practices and policies which you have agreed to adhere to, you will be wasting your time.
"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)