@SmillaJaspersen may understand this, but for the benefit of other new players:
We can see and take (fair 2 star) trades from other players only if at least one of the following is true:
(1) We have discovered them on the World Map
(2) We are in the same Fellowship
(3) The trade is for certain late game resources (sentient or ascended goods) and we've completed the necessary research and upgrades.
Typically a veteran player will have discovered hundreds of cities, including many newer players who yet haven't found them. Fair trades between such players can only happen if the smaller player posts a set of trades, and then the larger player takes them. (Since, fair trades are very beneficial to both players, most veteran players will hoover up any reasonable trade requesting their boosted goods.) Posting trades therefore greatly increases the number of potential trading partners for newer players.
Edited (following
@Solanix's helpful clarification below) to specify that the above relates to 2 star trades.