In LiF: YO we need a complete system of trade, including currency, trading posts and the ability to exchange goods between players.
Money - gold, silver and copper coins - the right of coinage to provide players with high titles, for example, look at how it is implemented here
http://lif.mmohost.ru/viewtopic.php?id=3#p3. Or some items in the trading posts have to buy AI.
Trading posts should work on the principle of the auction. Sellers can name their price, leaving the goods for sale. The buyer, respectively, will be able to choose a product on price and quality.
For exchange between players must trade menu, which will allow to carry out the transaction only after the approval of its terms by both players.