So aside from The NPC Market which you can find the link to on the "Information" sidebar on the right, you're entirely looking at trading with other players in the game - not just hanging out in places where ships might be.
The Commerce Forum is an on-site place for traders to advertise their wares (including ships) and for traders to post "WTB" (which stands for 'Want to Buy') requests to have traders with assets (e.g. ships) contact you.
Trade Federation also has their Trade Federation Marketplace aka the DoT Marketplace, which they run on their own site. It is very much like a classic online shop which you may be used to from other circumstances.
Centrepoint Marketplace is primarily an Auction and Sale site which has been a long-time staple of Star Wars Combine. Remember that when signing up, you should talk to Guybrush Threepwood to have your account approved.