Trading guilds bid gold weekly to hire the traders, amounts vary based on area, some traders cost a couple hundred thousand a week up to multi million for the busier areas. There are plenty of free guilds, donation based guilds and guilds that charge weekly dues. It’s up to you what is best for you. The free and lower cost guilds are in areas with less traffic, that means less chances for you to sell your stuff. You can stay in guilds or change them as often as you’d like, there are no rules about that. We have no central auction system, this is the system zos created for us & while it’s got its own issues, it does function as intended, for the most part anyway.

There are guilds you can join that don't require weekly fees, and you can sell in them. They may or may not have a guild trader (I know of one guild that does all of their sales in-guild only)

If you don't want to pay guild dues, you can also sell things through zone chat

Every city has different trade guilds to increase the number of guilds that are able to have guild traders. You can travel to any city at any time, so you don't have to switch trader every time you switch cities.

You can access your guild's store from any bank in any city- the only advantage of having an actual trader is that it lets you sell to players outside of your guild.

It's a massive pain in the bum, is what it is, but to say so calls down the wrath of the Elder Corps. Also boring dead horse memes and gatekeeper frenzy.