Ideas.
(1) Shopkeeper funds -
Shopkeepers can only spend money they have. Each shopkeeper has [X] money, and this number slowly increases over time (from assumed npc purchases). This means that you might have to find a rich man\'s shop to sell an expensive item or settle for a low lump of cash. Additionally... each shopkeeper can onlyhave [X] much money maximum
Option: The less money a shopkeeper has, they less they are willing to pay for different things.
Reasons-
Limiting the speed at which players can reduce drops to liquid assets will in turn help inflation.
Possible methods of implementation-
Method A) Simply add 4 to the shopkeeper\'s cash on hand every half second/second/whatnot, and cap it at a certain amount. Higher-class stores (magic stores, in particular) might have higher caps, longer cash-on-hand increase waits, and larger individual cash-on-hand increases.
(Ex: A general store owner might sell bread, wine, and general supplies cheaply; a relatively constant income of small amounts. A potion store owner might only sell only a few things each minute, but gets more cash from these sales.)
Method B) When the total value of items in a shopkeeper\'s inventory/shopbox reaches a certain value, they no longer buy items. Possible graduated scale (Ie, when value reaches 60% of cap, pay 10% less for items, 80% of cap pays 20% less, simulating how less money on hand would make the shopkeeper more reluctant to pay out money). I\'m not sure how you\'d code it, but I think this one might be easier.
(2) Total Cash Available
It\'s entirely beleivable that there might be more value of goods in an economy than there is of actual gold/silver specie (as this case may be, crystal specie). To increase the number of raw currency, you have to go mining of course. Possibly take the total amount of money available to shopkeepers worldwide, and limit it (Ie: 10 shops with caps of 10,000 currency each, but the sum of all of their currency can not exceed 50,000).
To increase the amount of money available, mine the crystal/mineral/etc and take it to a mint/gem polisher/etc and get it turned into viable currency. Every times a character does this, increase the amoung available throughout the world a bit (Ie: Joe mines 500 currency worth of raw material. The total currency available for the shops goes from 50,000 to 50,050). No idea what would be good to balance, but 5:1 or 10:1 should be a good starting guess for test purposes.
(3) Player based economies
Possibly, cut out stores entirely. Listen me out before shouting me down. Instead of stores, have \'sale houses\'. Players put an item/group of items for sale, but don\'t collect cash until another player buys the item/set. Basically like an open market, but simplified to make it easier on the players - instead of trying to buy and sell at the same time, you set what you want to sell, brows what others are selling, and then check to see if your items sold before leaving. If not, leave, quest, whatever, come back; if it\'s sold, then collect your money and move on.
This is probably the least susceptible to inflation, since the only way of actually increasing the amount of liquid currency in the economy this way is through monster drops/quests/game-based methods. Very easy to control cash flow. If there is too much cash lowing around, though, just have \'NPC sales\' - players get an item, but the money goes to an NPC (dissappears).
Also the most accurate for supply and demand. Items with high demand can fetch better prices, low demand means low prices (or, for extremely rare but also not frequently used objects, massiely high prices).
Could also lead to a complex barter system - trading items directly for items.
If I may use an example from another free MMORPG I\'ve played (Runescape), the object-barter used to be almost a rule. Coal certificates (worth 5 pieces of coal; used in smithing) were almost universally worth 1,000 gold. People would frequently use coal certificates as a means of paying for other goods (high-end equipment, large numbers of magic components/other consumable resources, other types of smithing material, etc).
The certificates themself also served a purpose - coal is the basis for all high-level smithing (steel, mithril, adamantite, runite). A \'consumable money\' like this might also help limit inflation.