My gold chain falsely failed the authenticity guarantee check

I recently sold a 20" gold belcher chain and sent it off to be authenticated. It failed thecheck with them saying 'The item is not hallmarked, or the hallmark is illegible'. I just told ebay customer services that it IS hallmarked and they dont care, instead batting me off with some pedantic definition of a 'hallmark'. No hallmark heh? Its as clear as crystal.

Don't sell jewellery on ebay, it will definitively fail the check because ALL hallmarks look like this

HallmarkHallmark