this can be done in one of 2 ways
1 ) add Vendors that will buy Resources for say Silver and have another Vendor to sell Resources for Silver
2) add in a auction house were people are able to place buy and sell orders for resources with gold as payment. like buyer 'A' puts a buy order up for 500 wood. he is willing to pay say 10 gold for that wood. so person "B" comes up sees that person 'A' wants wood and is willing to buy it for gold. he accepts the trade house offer and he revives the wood in his Trade house inventory (so he does not get encumbered) and then moves it to where he needs it.
obviously they can be tweaked to suit the devs but i think this would be good.
