that's a good point. we will discuss this.
As a suggestion from the peanut gallery-- You have about a half a dozen global factions. How about, one market per global faction where:
-The item 'base-price' to buy varies +/- 10% from the global 'list-price' set at the beginning of the game, per global faction.
-The item sell price = regional 'base-price' - 15% (Since the item is 'used', the vendor needs to make a profit, etc.)
-Keep roughly the same number of items in the initial market as there is now (since your base is only in one place at first), but retard the 'item growth rate', as there will be per global faction markets.
This:
-Limits programming overhead, since you're not writing a global trade simulator.
-Encourages item transferring of 'cheap' items, but the 20% variance in 'base-price' vs. the 15% markdown on sell price makes it hard to turn much in the way of a profit through simple trade. (You'd have to buy 'way underpriced', then sell 'way overpriced'.)
-Encourages players to build a global network of bases.
Edit: For brownie points, once a month you could give each item a one in ten chance of changing value by +/- 1%, but be bound by the +/- 10% maximum variance. (The drunken sailor example says that prices shouldn't run away, but...)