People with frags, colonies, livestock or other reef related stuff post their good for sale or trade. You can look at their good and request something you like. They will contact you if your request if accepted. You can offer trade by e-mailing them. You need to register to be able to sell or buy. Work pretty good. Read CORA's thread on fragswapper rules.
HTH
Please feel free to add anything that I might have missed or was incorrect about.
Phil