Yes I totally agree with what your saying, but based on people it wont work as well as you think. (Awesome that you brought up contracts because thats exactly how I picture everything. Every deal, trade, quest, etc).
Lets say I post a quest asking another player to gather specific items from a specific monster. Again this brings up "items" and with items why not just go to the market and buy it instead of paying someone to go get it and have to wait when you could get it instantly. It would only work if it was out of stock or something. Lets say a player chooses to participate in the quest. Is that the only player that can participate in that quest? Is it up to the player that gave out the quest to choose the player amount? Okay lets say the player that took the quest begins the quest and then stops halfway and logs off the game. Does the quest automatically becoming void? Will every contract need to have a time limit? Lets say that every player given quest will require a time limit. What if 3 players all fail before they finish, they player that gave out the quest will have to wait for the 4th player to finally finish to get what he wants. What if you give out a quest and a player takes it and then you find out halfway through that you don't want those items and don't want to pay the player for finishing the quest. Will you be able to cancel the quest halfway through making the player that took the quest angry that what he did was for nothing? Will the player that gave the quest just be screwed and have to pay the player anyways?
I've thought this through a lot and the only way to do it is to make it simple like I said before. A way to make it work better would be to not have an in game market at all, and I'm sure players would hate that. Even then players would just trade personally and go onto forums and the like in order to trade instead of dealing with quests.
Everyone wants instant satisfaction instead of having to wait for what they want.
EDIT:
I guess I'm not saying that there isn't a way it could be implemented in game, just that there is no point in implementing it when there are much easier and quicker ways of getting the item.
I believe here the point is make the item, and economy of the game, as coerent as possible. One fun implementation I just saw on the MMO Path of Exile, is that there is no money/gold in this game. Every piece of loot has a price in scrolls and some new kinds of items that have the ability to empower your gear. I can't say yet if that economic model works, but seems fun and true enough in a fantasy enviroment.
Another point is, quest giving and time. One game that has a fun loot system and great quest design is Monster Hunter Tri. There is no trading though, because it would make questing/looting pointless, since the only way to "get stronger" in this game is loot (there is no character level progression), one thing that Diablo 3 failed to accomplish. In MH3, for instance, quests, all of them, have time limit. Every single quest has a 50min time limit, and that feels just great. While some quests need no more than 5-15 min, many times you can fail at the time limit or finish the quest at 45-49min. And that quest design is good in many ways. When you log in to play it feels like a "round" of game, so if you have less than an hour to play you can just make some short quests. If you have an hour make one hard-long one, and you feel you accomplished something on that time.
That quest design just fits into a "player as quest-giver" enviroment and also gives players the feel of fullfillment for a short ammount of time played. I believe most MMOs fail at this point with it's repetitive quest design and grinding of level (or loot) at high levels of the game. And even if you have a good "arc" or linking between quests you can give players a feel of "campaing" (MH3 delivers that also on the single player campaing) with this time limit.
As player I even have this problem when logging out-in. Sometimes I get like "wut quest was me doing anyway?" ¬¬ Maybe that is because of the repetitive design of quests. But again I don't think the flaw is in the design of the quest anyway. I think the flaw is in the way quests can be accomplished, most item quests are designed around the kill-and-loot concept while that can be fun sometimes it becomes boring after you've done it a thousand times.
An item quest could be designed to be accomplished in various ways, that would customize gameplay. Let's think about real life for instance, If I am hungry I have several ways to accomplish that quest. I can go to a pub/snack house or restaurant. I can go to the market, buy some materials and cook my own food. I can order food at home. I can look into the fridge and eat whatever is there. That is what gives me personality and meaning, the fact that I can choose the way to do some thing. I can choose a cheap way, a quick way, a delightfull way of doing that and all of them can be fun to me for different reasons. I can even train some skills (like barter and cooking) while doing that "quest". An item quest could be designed the same way. I can steal that item from someone, kill and loot, craft it (maybe even craft a cheap version or fake version of the item), buy it from some vendor (maybe even steal it from the vendor), but all that needs to be meaningfull. If the item is too cheap in the market the quest doesn't even make sense, since in a "real" enviroment the vendor would already have reached the quest giver (or that would make the "buying from the market" option mandatory).
But the point here is giving player roles (or playstyle) some meaning on the task. What classes have your game? What kind of resources they have to accomplish the quests? In this enviroment it wouldn't even hurt to have some quests being impossible for some classes or too easy for others. While most MMOs focus on combat that quest design would give players the option to never get into battle if that is their playstyle (and that is an option I'd like to see implemented on MMOs).