Looking into the parts that guy sells, he seems to be using a stepper motor which needs a circuit board to 'pulse' to give the correct 1rpm...
Yes, he uses a stepper motor with a circuit board. If I understand correctly, the rpm of a DC motor might fluctuate as the batteries get weaker - or it gets colder, whereas with a stepper, the rpm is constant until the batteries just plain quit.
He uses a quartz timer for the stepper which is very accurate.
For comet Hyakutake, I built a manual only barn door tracker. I then built a DC motorized tracker for Hale Bopp. I had to monitor and adjust the rpm carefully. Now with the stepper motor, the rpm's seem to be more consistent.
BTW, what parts have you found on ebay?