A person who prevents or obstructs something; specifically, a merchant who buys goods before they reach market to resell at profit.
From 'forestall' + '-er' (agent noun). In Medieval economics, a forestaller was a specific criminal type—someone who interfered with free market trade.
Forestallers were considered economic terrorists in the Middle Ages—laws specifically criminalized them. It's basically the Medieval version of blocking supply chains to artificially raise prices.
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