Faucets and sinks
Also said as Sources and drains, Gold sink, Money sink
A faucet creates a resource out of nothing, as when monsters drop gold. A sink destroys it for good, as when you pay an NPC for repairs. Money passing between players is neither: it just moves.
Every online game's economy is decided by these two numbers. If more comes in than goes out, money piles up forever and prices follow. Designers have to keep inventing things worth paying for, just to get the money out.
Say it
“Daily quests add 400 gold a player and nothing removes it. Add a sink: repairs cost 5% of what the player holds, so it settles instead of climbing.”
Where you’ve seen it
EVE Online publishes a Monthly Economic Report with its sinks and faucets listed, so players can see how much ISK came in from NPC bounties and how much left through taxes and fees.
Not to be confused with trading
Selling an item to another player moves money and creates none. Selling it to a shop creates money, which makes that shop a faucet, and it is easy to forget this.