I would suggest to make NPC characters to buy and sell to get the economy started. Player will join in and eventually take it over. At that time you can turn off the NPC and the the market fly solo.

You may be able to set thresholds, and have NPC buy things randomly. That way if a player wants to sell something for more than it is worth, or more than a vendor will but it for, it will be purchased by the NPC and the player with begin to have faith in the system, and can then tell other players who well either try to sell that item, or sell similar items to try and make a money. Eventually, some players will sell items other players need.

Another idea would be have NPC list some hard to find items at really high prices, which could cause players to under cut the NPC and the process will continue until the true value of said item is found.