Well, many games that uses curency, specialy long term games like MMO is suposed to be, have a great problem in the long run, inflation, so to solve this problem i would build curency system in this way:
1) there would be some NPC trade posts around the map
- this NPCs would sell and buy itens from players, but, if you sell something, for exemple, 1 iron bar, you would receive 1 copper coin, if you try to buy one iron bar, it would cost 10 copper coins.
2) alow players to forge curency from metal, and metal from curency.
-is just a question of balance, for exemple, if a copper bar generates 2 copper coins, a copper bar on NPC should cost much more than 2 copper coins, same for silver and gold.
3) if a guild want to trade, it can build a trade post, there it will items they want to sell, and the price they want (up to a maximum Wheigth, or even more than on kind of trade post with diferent sizes) and could put auction on items they want to buy, and the price they are willing to pay.
4) the trade would be made for players, where the player should acess a NPC building, (trade post, tavern, etc) and ask for "merchant rumors", this way it would see a list of offers (buy and sell) in the area, and could mark a pin in his map, and go there to sell or buy what he wants.
Overpowered NPCs tend to inflate the prices, at the same time when you let the players choose the prices they are willing to pay for some items, they will generate offer and demand, this wil be even geographical in the game and will simulate rela life.
maybe some players do not want to travel long distancces to buy some item, but other will.
Sorry my bad english