I actually saw an egg cup at the thrift store today. It looked maybe depression era glass...maybe i should have bought it, it was only 49¢ but i am not an expert at glass and i don't think there would be much interest if i tried to sell it as i know of no one that uses egg cups and do people want one offs or do they come in sets?
You can buy them singly or in sets here - there are people who specialise in collecting egg-cups and they are certainly interested in individual items. I'd like to have seen the one you mention.