A few thoughts, forget my rusty english.
The first thing to understand is that there are no perfect trading systems. The second thing is to define what we are trying to do and what principles are behind:
- For me, the trading system must be based on the principle of freedom, the less restrictive as possible.
Objectives
- Players able to sell their products and get profit
One of the most important ends in mmorpgs is the sense of economy progression. Players don't play mmorpgs to be poor; while the money is a mean to get things, the end is always being better than the starting position. However being rich is also problem because it makes the game too easy.
- Not tedious or restrictive
A system that requires many requirements, forced guild membership, excessive gold sinks will encourage:
a. direct trading: chat spamming, subjective sense of the products value (even scamming)
b. players who won't be trading at all: some people will skip trading if they feel it's not worth the time and hassle.
- Nobody controls the economy
A global AH would makes a few to control the economy by buying low and selling high, creating monopolies of resources. So there it is important that Auction Houses should not be connected. That's why I think local Auction House could satisfy the same guild stores purpose in this matter.
- The system is adapted to the size of the server.
Too many players and the almost infinite supply, generates a fall in prices. Selling wares could not be worthwhile. This, won't be a problem at the begining (see below).
- A system that evolves with the game
At first, the product prices will be high, this is inevitable because there will hundreds of thousands looking to buy materials. However, as the game gets old, the price of the lower level products will be devalued because there will be less demand (the supply will be less too). It is also inevitable.
/edit
P.S.
There are more effective ways to prevent/limit goldselling.