There are two main types of trading being discussed here: trading in the Galactic Marketplace, and trading with other characters.
The Galactic Marketplace is part of the Darkness interface, and can be accessed through the 'Market' link in the Members Menu on the right side of your page. You can view what is currently for sale in the Marketplace at any time. However as the Rules
say, "To buy from (or sell to) the market, you must be inside a Trading Faction owned
market, which means inside a Commercial Centre facility or a Trading Station (1 or 2)
, or inside Centrepoint Space Station." Here you will be able to buy entities that other players have already sold to the Marketplace, entities of players that have died or entities from factions that have dissolved.
The other type of trading is where you trade directly with other players. This can be done by arrangement through DMs, IRC, faction forums or CPM which is kind of like SWC ebay.