No, autobuys are perfect, they have no running costs - but you as a player can generally buy a lot of something when the price is low and live with that, and that is more effective cost-wise.
If you have 3 engineers, you can automate autosell and have everything running well, if you decide to drop coal or oil. Save the one that gives the most profits; the rise of selling efficiency to 100% should be well worth the drop.