No info on how to join a guild that trades a lot and usually maintains a trader?
If you want to join a big trading guild, best thing to do is keep an eye on zone chat in one of the higher populated cities. You can also directly contact someone in a guild you want to join. Just send a message to one of the sellers that are online and ask if they have an opening and if they can get you an invite. I am in 4 large trading guilds and occasionally get someone messaging me. If there are any openings, I usually try and get them an invite.