I find it hard to believe a company could operate legally without a complaints procedure in place?
Businesses are not required by law to have a complaints procedure, and eBay doen't have one. You can write to their head office address in the user agreement.
As for regulatory bodies, eBay's handling of payments is regulated by the Financial Conduct Authority, but eBay has structured it's business in such a way that its money back guarantee is unregulated. There is no right of appeal to any independent authority.
p.s. for the sake of completeness, I should mention that eBay DOES theoretically have a complaints procedure, but it isn't published and as an eBay rep. told us a few years ago "it cannot be used for appeals of suspensions, restrictions, eBay cases or a lot of other things." In other words, it can't be used for the very things that users most want to complain about!
