They're only allowed to fill trades at all if they have a specific order from a broker. And when they get one, if the price isn't what they'd like it to be, they don't have to fill it.
Once a formerly-suspended stock begins trading on the Greys, usually the first trade is very low. Going along, the trades will move a bit higher, but the price will remain extremely volatile. Generally speaking, volume is highest in the first few days after the stock resumes trading. Eventually, a range will be established, and volume will die down.
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