[Cupid]

I think once you get to the stage where you can't link your account at all your only option is to contact Mario direct via email at the support address given on the Contact page.

Ultimately though the linking/authorisation procedure is something that's provided in conjunction with Ebay, so if the issue turns out to be on their side it might be that Mario has to report it to them and wait for them to fix it. He'll need a good example, like yours, to be able to investigate further before he is able to do that though.