Goods being sold in Marketplace.

[Sinister Soldier]

The thing is, NI has a constant, steady increase in gold. Every kill gets us some gold, and the potential to change our exp to even more gold. If there's no way for this money to disappear, people would end up having masses of gold, while all items traded between players will cost you more and more gold, just because, where before you were rich with 1m, you would later be rich only when you reach 20m, so whatever people value high will constantly increase in value to no end.
Adding materials you can only get through cash creates both a sink for cash, so that money doesn't always stick around, and gives us a way to anchor the value of items that use such materials.

This ^