Most major trade guilds charge a small portion of money each week in order for you to keep trading with them. This isn't for them to make profit out of having lots of members, it's in order to keep the guild going and continue pumping money into the guild traders.

I'm with Fair Trade, it's 3000 gold per week to join but it's worth it for the sheer amount of members in the guild, chances are you'll sell most of your listed items within 24 hours. I've sold at least 20 items in the past couple of days just through the store.

Head over to the Guild Recruitment section of this website and have a look around, depending on what platform you play on and server location.

Most guilds that don't charge to sell with them usually only have a small amount of members, so selling items will be much harder. Also check Social Media/Facebook pages for trading sites, also depending on your platform/server. You can list item's on there is what you're selling is rare/pricey.