In your position I would write (with proof of delivery) to eBay's head office address in the User Agreement.

eBay's payment system (unlike its money back guarantee) is regulated by the Financial Conduct Authrity.

Even a threat of referring your complaint to the financial ombudsman could be the motivation eBay needs to sort it out. Replying to  the ombudsman would require eBay to use staff time to prepare a full response - not a simple job, especially if you can document the 50 times you say you have called them.

It can be a bit of a game of bluff! eBay certainly wont want the work and possible reputational damage of a complaint to the ombudsman. But you too will do better to resolve it with eBay if possible. The ombudsman would not consider whether eBay's behaviour was "fair" to you, only whether eBay has correctly complied with its own terms and conditions. However unreasonable it may seem to you, they may be acting within their rights. You would need to read the user agreement carefully first to see what powers and time limits it gives eBay in this situation. It may also be relevant if you can show that eBay repeatedly promised actions which have never been taken.