I'm using Sythe.org as seller for few month. I never had any issues with the forum (Trading/Staff), I highly recommend it for everyone looking to buy or sell in-game items.
I don't only use Sythe.org for the marketplace, but mostly for the awesome community. I have learned quite a bit from a lot of the members here on the site. Most of the people I've met are super kind, friendly and helpful! This is one community I do recommend!