I use the Android platform, and I have 3 cash registers going at once and all are being used by customers. I had the problem that gczilla had, but I just kept moving and rotating the registers until I got a person to show up....then they worked fine. I think the key is to be certain there's enough room for customers to get up to the register and also for there to be room enough for the person to stand behind it. Rotation seems important, too, somehow...not sure why. I hope this helps you. Selling the register will get you next to nothing in return, so I'd try this if I were you.
Best wishes.