you will always do the trade itself on the market place in ladder slasher, so mediators are not necessary.
if you sell an item in a thread here in the forums, you contact the person and put the item on the market place for that amount of fg.
from the moment it is on the market, anyone else can potentially buy it, too.
so you both should agree on a time to reduce the time it is on the market till the person who won in the trade buys it to the lowest possible.
this guide and guide picture will explain how to use the market place and explain features like item fee and such:
http://forums.d2jsp.org/index.php?showtopic=8108466&f=272