I'm from America where we believe in free trade, so what I'm about to propose may be considered crazy by others.

Why not trust the economy into the hands of an NPC? You have market wards that each contain items in their proper places sold by NPCs. In reality, these are items that are there from PCs.

Now, there is also a seller's counter. You can sell any item to the NPC behind the seller's counter and get the gil RIGHT AWAY. The NPC buys it from you, then tries to sell it in the market wards (for roughly a 10% increase). Through this, SE can regulate prices in the economy. More so, they can apply a supply and demand formula. Too many wildfowl feathers? Price goes down. Iron Ingots selling like hotcakes? Price goes up. If price goes down too much, you can always vendor the item.

So what about items that are with a lot? This is where the unique tag comes in. Instead of being sold to an NPC, there will be another NPC that will put these to auction for a price the player specifies. Only unique items can be sold on an Auction House. This is a REAL auction house that you can walk into. Auctions are held on the hour every hour (and a list of items for auction will be posted outside the auction house with the starting bid). Before an auction starts, you can silent bid on an item (which in turn will raise the starting price on the list). If an item receives less than 7 silent bids when the auction starts, the item is immediately sold and can be picked up from an NPC. If it has 0 silent bids, or 7 or more silent bids, then the item goes to auction. Items with the most silent bids debut first. Items last on an auction house for 48 hours before being removed. An auctioneer fee is paid if an item is sold.

Pros:
-No more search for retainers, simply walk through a lively market place and speak to an NPC. Purchase the item and you are good to go.
-RMT cannot control the market.
-Every craftable item will be on the market. Because when that item is not on the market (no matter how well it sells), it is always profitable to buy the materials off the market and then sell it to the NPC to list it on the market. The return is instant and it always nets profit. Theoretically, a player should then always be able to find an item in the market wards, given that the materials are always on the market wards.

Cons
-If you make your money by finding deals or buying out and raising prices, then you lose your money maker.
These ideas combat RMT as RMT will not be able to control prices (after all, they are controlled by SE).