Well, villagers do trading so you have a basic economy there.
But you know, this is a real good idea.
Say a mailbox item that opens a GUI with a long scrolling list of trades you can do, created using the same trade system the villagers use. Pick a trade and how many times to do it, drop in the required payment, and when you hit send it enters the outgoing queue. Every minecraft day at a certain time the outgoing queue gets processed and the results enter the incoming queue (which gets delivered the next day).
GUI work mainly, a tileentity, a bit of scheduling, proper saving of the relevant data and you're done.
Biggest problem would be mod items, but you could easily add a config file to let people list mod items and their relative cost.
From there you could have fun with economic theory and do whatever you wanted for simulations in the background, maybe add special events that put items on sale or jack the price up due to global shortage. Maybe add a commodities market tab so a core group of base resources are always available, though at prices that fluctuate based on player and simulated trading.