Are there good two player rules for resource trading in Settlers of Catan?
While I've never tried it out, there is a variant set of rules here that uses the white player as a neutral third party.  It looks a little interesting, but the rules in the link only allow the neutral player to build roads, and nothing else.  Building roads for the neutral player is determined by highest dice roll.

It does allow for trading with the neutral player using a special trading chart.  The chart may or may not need to be tweaked to your liking.