Market Discussions

Honestly, none of them. Your time is better spent updating orders, looking at contracts, or even doing PVE. The majority of the people in those channels are traders like yourself and looking for a good deal, so unless your prices are low enough that someone could resell your stuff for a profit then most of the time you'll spend in those channels will be wasted from a sales/profit point of view.

That said, with the increased market taxes/fees perhaps more players will start using these channels to sell things again. For chat channels there's the official trade channels (ships, blueprints, modules, etc.) which you can find in the chat channel list, occasionally others pop up, but those are the biggest ones. For mailing lists the only active ones I know of are Wholesale Trading and Concord LP.