Let the people choose the "currency" such as the wastelanders in Fallout used caps as a store of value.
This actually reminds me of what happened in Guild Wars. Guild Wars had gold (only, no silver/copper), and you were limited to carrying 100k gold on any one character (bank stored way more I think).
The thing is, some items that were being traded started becoming more valuable than the 100k gold limit on a character. Since no one wants to do a trade in multiple parts, because of how insecure that is, people started using a very rare crafting item that was dropped in a specific area as currency replacement - the item was Glob of Ectoplasm.
Globs had variable price, even at NPC traders (the prices varied there based on supply/demand), but even so, people took to using them as substitute for large transactions of currency, since the supply of ectoplasm was always strictly limited (it was hard to farm), and also since it was a consumable, valuable item - so that the market wouldn't be overrun with it as a supply.
I'm pretty sure that mmo economics is a field of its own, since it can become incredibly complex.. but having a rare, limited in supply item could just lead to people using it as currency naturally (kind of what happened with gold in real life I suppose)