The website poe.trade is very good for finding, buying and selling items in Path of Exile, and most people who trades uses this website for finding and posting items.
The easiest way to list your items on poe.trade is to buy a Premium Stash tab. This allows you to make the stash tab public. After it's public you can right click on any item in the tab to add a price tag to it. This lists the item for sale on poe.trade with the price tag you put on it. Now anyone can search for the item there and whisper you if they want to buy it.