First up, RING eBay and request to speak to a supervisor. Or, you could request that one call you back if none are available. I believe they are pretty good at getting back to you pretty quickly. Explain what you have written here, in regards to measurements etc. Get them to open your listing to check it and also get them to check the messages between you and this numpty, which they can do. With a bit of persistance (it might take a few calls), hopefully they will reverse the decision and the turnip will have to pay the return postage.

This is absolutely outrageous and shows just how ridiculous this new MBG is. Please keep us updated on what happens because a lot of people either have been in, or will find themselves in the same situation sooner or later.

Good luck!