The broker typically places what is known as a " nominal order " which differs from the standard market sell order that one would place when a stock has a zero bid. When a stock has a zero bid, there simply is no one to take the other side of the trade when placing a market sell order. But when your broker routes the order as a "nominal order" , it is basically a courtesy request for a market maker participant to take the other side of the trade at the value of 0. So he is essentially purchasing your shares at .0000001 just so you can close out your position and take the maximum tax loss. The market-maker now holds your stock in his inventory and you no longer hold possession. The downside of course is that he will be the beneficiary of any money made in the event this particular stock would increase in price in the future. Basically he bought your worthless stock for nothing so his risk is nothing, and you get to take the capital loss as a realized loss against your realized gains.
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