'Pumper's' exist in many penny shares to boost the SP so that other's can make money. It is usually 'arranged' by the BoD so they can sell their own shares quickly. Once this happens, the SP will go back down again before other seller's can sell off all or some of their holding.
Invariably, you won't be able to sell that many in one go, it's nearly always in Tranches and there can also be 'Negotiated Trade' which could be minutes or even hours before they get around to buying yours. By this time, the SP can decline dramatically, but it could also increase, although unlikely. It's often a ploy run by Market Maker's.
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