Big investors set up sell walls as a way to accumulate their stakes without increasing the short-term price. A sell wall results in an oversupply of the market. Consequently sellers trying to liquidate their assets have to sell below the price of the sell wall. 

Howsoever, the volume of sell walls always exceeds the regular demand of the market. Respectively, the price will eventually go down as long as the sell wall has not been bought up. During this period big investors subsequently accumulate their stakes without increasing the equilibrium price. The opposite of sell walls are buy walls.