That is not an eBid rule; it is a piece of information in a help file drawn from UK law. US law may differ.
Even if eBid did hold the seller 'accountable' - then what sanctions do they have? Only to prevent the seller selling on the site. They are not a party in the transaction, they do not handle the goods or the money.
All eBid do is provide a service for a fee. If you rented out a shop would you, as the landlord, be responsible for paying compensation to any dissatisfied customers of your tenant? Would you feel obliged to seek redress on their behalf? No, of course not. If a dissatisfied customer cannot reach an amicable settlement with the seller then their only recourse is law.
I am not lessening your complaint. I simply feel your expectations of what eBid can and should be required to do in such situations is unrealistic. (eBid is not, I don't believe, in the business of sending out hired goons with baseball bats to extract compensation)