Here's the thing that I don't like about the typical implementation of global chat channels...
People never use the channels for what they're intended for.
People looking to trade or sell or buy something will typically just use whatever is the most active channel and spam it to get it in front of the most eyeballs.
People who have no reason to be in the trade channel will be in there talking their heads off about everything but trading/selling/buying.
"Just disable the channel if you don't like it" should never have to be a suggestion to deal with it, because then you're missing out on discussion you might actually want to follow or want to be part of. So, the solution of "just disable the chat window if you don't like it" should never be a required, nor suggested option.
/blist-ing people? Perhaps, though again.. not really an elegant solution to the issue and more of a band-aid to deal with a less than ideal one.
I've wondered if a system like this could be useful...
For trade, only trade-specific messages will be processed. And how do you control this? By automating it.
For example, you could tag items in your inventory as something you want to buy/sell or trade, and then enabling a sort of "advertise" toggle. If you don't have the item in your inventory to tag as a "WTB" then perhaps a system similar to the auto-translator where you locate the item that way and select it could work.
You can enter a custom message (like the search comment function in XI). Enabling it will basically automatically post an update to the trade channel on your behalf in certain intervals... like every 2 minutes or something.
The itmes you're buying/selling/trading could show up as linkable items, so people can click on them and have the item's stats show up in a pop-up window, much like you can in some other MMOs.
No regular discussion can take place in that channel, you can not manually enter messages to it. If you're interested in discussing a possible transaction with someone, you PM them personally and discuss it that way, or group up and talk in party chat or something.
Further, trade chat can only take place in the trade channel. Filters would be implemented to prevent, or at least greatly reduce, the effectiveness of trade-type messages in regular chat. This would be the much trickier part, I think, as you know people will always try to find ways around it. But I still think it would help more than hinder.
This would be beneficial in a few ways..
1. No spam of WTB, WTS etc filling up the chat log - especially with how often some like to spam them.
2. People don't have to either manually type or even use up macro space to post the messages.. It happens automatically for them.
3. Trade-specific messages stay where they belong, keeping the regular chat channels specific to regular discussion.
I think a system like that could have merit if well implemented.