What would really happen is that a second trader would enter the market, offer to buy the laptop for 110$, hold onto it and then sell it on for 140$. Actual prices vary and with enough competition, will settle to something that gives the trader still a nice profit and enough cash to insure against the risk of such an operation (the trader doesn't know up-front when he can actually onload the goods and how far the price might move meanwhile).
Of course, if you're not desperate on selling right now and or the risk for the trader is too high (i.e. the minimum price you'll sell right now is higher than what the trader is willing to offer you right now), nothing happens.
Concluding, traders do add utility to a market. And as others have already said, with high freq trading it mostly results in massively reduces spreads (and the temporal utility as outlined in the laptop example essentially disappears).
Of course, if you're not desperate on selling right now and or the risk for the trader is too high (i.e. the minimum price you'll sell right now is higher than what the trader is willing to offer you right now), nothing happens.
Concluding, traders do add utility to a market. And as others have already said, with high freq trading it mostly results in massively reduces spreads (and the temporal utility as outlined in the laptop example essentially disappears).