This is the how to:
Trading is just that...It is a trade. So look around and see what an eel your size would cost.
Put up your ad saying FOR SALE or TRADE, then when someone contacts you they will make an offer on whatever they have to trade. You decide if the trade is worth it.
You always have the option of trading straight up, or getting an item + money...or someone may just offer money , then you buy what you want with it.
Happy Reefing