If this happens on an exchange (such as Coinbase), there is no risk, since the trader/middle man knows that there is a buyer for 140 and a seller for 120, so he

If this happens on an exchange (such as Coinbase), there is no risk, since the trader/middle man knows that there is a buyer for 140 and a seller for 120, so he can 'front run', buy at 120, sell at 140, keep 20.