Fun, perhaps a different targeting system would help with the difficulty. I also ran into issues with a cart not firing when I had selected a target it seemed. Indicators of what you're doing would be very helpful (highlight cart when selected, highlight target when mouse over/show a highlight around them to show they're being targeted, for instance). Shop could be completely redone, no need for a buy/sell mode. Just show prices and let us drag around all in one and it'd be far less confusing.