Um, the same way a bank prevents its employees from debiting $1,000,000,000,000? Can't spend more than the banks bank?
But less sarcastically, they again work like a casino. And like a casino must do, the company keeps on hand enough money to "cover all jackpots" no matter how unfeasable. They claim they only take profit from item repairs (cash sink) .... the money spend on auction fees, ammo, npc purchases and sales, basically anything not repairs is simply recycled. Thus me shooting 1 ammo is like me putting money in the slot and eventually all those shots add up to a nice pool for someone to get a nice kill/craft/mining result.
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