Examples of board games with good/interesting rules for trading?
Settlers of Catan is a classic game with trading elements.  Each player needs to build roads and buildings with various resources (wheat, clay, ore, sheep, wood), but most players will have scarcity in one or more of the resources needed; thus, they must trade resources with other players to be able to complete purchases.

The key is that you need all of the resources, at some points, to win the game.  There is no strategy where you can get by on just three or four of the five, so you must trade often to win.

Often times you will see resources traded at uneven ratios.  A few groups add more advanced sorts of trading, such as futures, but those are not allowed in the basic rules.