How to define "to the best of your ability"?
In the rules for Harbor, under "How Buildings Work":


  Some actions have two separate parts which are not dependent on each
  other, and in those cases you may perform the parts in any order, or
  perform one part and not the other.  However, you must perform both if
  you can.  The actions with two separate parts are the Library, Bank,
  Traders Guild, Architectural Society, Privateer Ship, and Seaside
  Crane, and they all have the '&amp;' symbol.


So in your case you would exchange goods and then end your turn.