This place can help non premium member with trade, but dont abuse it.
You have two solutions:
1: You make a "shop" topic
Exemple:
Title of the topic: Your name's Shop [Edit Date]
Message body: List all the items you want to sell, buy or trade.
You can edit the topic to keep your shop updated.
2: A simple post to buy, sell, trade something
Exemple:
Title of the topic:
To buy: WTB XXX for XXX Gold
To sell: WTS XXX for XXX Gold
To trade: WTT XXX for XXX
If people abuse or don't respect rules, only first solution will be authorized.