Designing Multiplayer Economy around exploits and inflation

There was a lot of discussion about this sort of thing on Gaia when I was but a wee lad.

The main issue was that there were monthly limited cash items, and the cash items from when the game was young became extremely scarce and hence valuable. On top of that, money could be infinitely introduced to the system via minigames and posting. The result was that players who held on to old limited cash items could sell them for such enormous sums as to render every other item in the game practically valueless, which only further pushed up the inflation of said items. The problem was compounded when accounts holding said items became inactive or were banned for one reason or another, taking those items out of circulation.

Some measures Gaia took to remedy this included
-Taxing the marketplace and user trades: A percentage of the sale price was taken from trades and eliminated from the economy, hence lowering inflation. This was double-edged, as it pushed total item costs higher as sellers of big-ticket items attempted to offset the considerable (due to it being percentage based) trade taxes.
-Introducing desirable yet expensive items that were available only from NPC shops: By making items which have little resale value due to being infinitely available yet having a fixed price, then making those items extremely disirable due to their looks or functionality, a large amount of gold was able to be taken out of the system, reducing inflation.
-Recovering extremely rare items: Permanently banned and extremely long-term inactive accounts with owners who could not be reached had their items released to the public through lotteries, returning some extremely rare items to circulation and reducing inflation.
-Releasing knockoff limited items: Functionally and aesthetically similar items to the rarest ones were released in later cash shop limited months, which made demand for the rarer and much more expensive items much lower.

Through the above measures, Gaia was able to maintain with some degree of order an open market system with infinite capacity for inflation.

An alternate option that I had considered would be a "Gold standard" system, where there is a finite amount of currency available. Naturally, there would need to be a reason for extremely wealthy players to buy NPC items or otherwise pay money back to "the game", otherwise there'd be a situation where there was literally no money for newer players to receive. "Savings tax" and the like are possibilities.

Generally speaking in these sorts of games, the devaluation of earnings efforts is near unavoidable; the more money you allow players to introduce to the economy by receiving them as prizes, the more traders will be able to ask for goods and services (such as character art drawers, etc), and hence the less that money that's earnable through non-meta-gaming means will be worth.