At the end of the day it is an eBay Auction and feedback can be left if the item is misrepresented. A resolution to this issue should occur before any feedback is left, and by the sounds of things - it would be very difficult to source this backglass. The item is described as "complete" but is listed as a project - so there should be a few questions asked if you were thinking of buying this machine. Having said that - The machine is rare in this country, so I doubt if the seller knew that the game was missing anything ? But not replying to inquiring emails is strange.
Hopefully, things get sorted out in a timely manner.