The City Stock is maintained by the CWU; when a shopkeeper sells something to you, they buy something from the City Stock that has a physical value.
For example, let's say you buy 2 bottles of beer and the city stock is 50 beers. Meaning now the stock is 48 beers.
This also means that the city fund is filled up. Certin actions like wages, loans, rations, etc. will detract from the city fund.
The economy of the server is very interactive if you get into the more nuanced sides within the CWU or otherwise.