I'm in 2 guilds which I suppose you could call 'casual trading guilds'. They're both PvX social guilds but they usually have a trader as well.

Neither guild requires members to use the trader at all (or to pay fees, "choose" to donate to avoid being kicked or whatever else to fund the trader), just like they don't require everyone to show up for social events or dungeon night. It's there if you want to use it, like the other benefits of being a member. But they do report on total sales and top traders in the Message of the Day and weekly mail.

That means the requirements for a player to join are that they're a good fit for the guild as a whole, not just that they want access to a trader. Some people make a lot of money each week through trading, so I assume they're pretty dedicated to farming items to sell. But there's also a lot of people like me who just sell whatever extra items we have, and people who don't use it at all.

The one rule they do have (and again it's the same for both guilds) is you don't put WTS messages in guild chat - if you want to sell something to other guild members put it in the store. But I think that's as much about keeping trade spam out of chat as encouraging sales. They do allow members to ask if anyone has a rare item they'd be willing to sell, because there's no way to list things you're looking to buy.