What matters is the volume rather than the liquidity per se, but the two are generally pretty well correlated. The point is that moving a market costs money, making a trade moves the market against that trade, so even if someone is deliberately trying to move a market they'll pay more than they could ever hope to recoup. The exception is when there's a derivative market that has more volume than the underlying - in that case profitable manipulation becomes possible, as you can spend to move the underlying, losing money, but making more money on the derivatives where you'd bought the other side.