[edit: After some discussion, a better and different idea for the "Merchant Guild" emerged in the comments. Make sure to scroll till you get to them] I think it would be rewarding to create a "Merchant Guild" that is not just a cold, silent and (too often, manipulable by the dishonest) trading system like what exists with the Grand Exchange, but a sort of "magical contract" that can allow for safe and social Interplayer commerce to exist: a place where those with skills can contract with those seeking to buy and re-sell items for a profit, or those who seek the skills of a master smith, craftsman, or purveyor of any other ware that requires specialized skill (think Herblore, Construction, or Fletching) for a particular commission, can provide materials and/or payment to Player-members of the guild. Guild members could toggle their visibility and availability to accept commissions/contracts, and a system enforced by the magic of the contract can prevent scams, fraud, etc. (similar to the lending mechanic).
There can be certain "controlled items/substances" like certain invention items which can only be traded in certain quantities or on certain conditions that would balance the game, but otherwise, why not have a market that engages both buyers and sellers to do business and promote an understanding and development of good business skills and good social skills?
Guild members can have objective Reputation Measures visible to those who contract with them and can also offer a "surety-bond"--a penalty sum of money or other collateral--that would be put into the hands of the guild at the time of the signing of a contract, and surrendered to the customer if the Guild-member fails to meet the terms/timetable of the contract. This could even be a requirement for members who have yet to establish themselves as Reputable.
(Continued in comments)