Also, to add to previous ideas of a "cash shop" perhaps it would be worth it to take a look at Wurm Online's system, a game very similar to Xsyon.

In the store you can just buy money, thats it, just money. You don't redeem this money for items in a cash shop, its an actual game item.

The money in game has many uses. For one, all player owned "stores" use it. Secondly, to found a town, you need this money. This standardizes currency and gives an incentive to buy from the in game store. Player can just buy tons of money to pimp out a character or start a totem.

Its not a perfect system, but it works.