I saw one of these in action today. It occurred to me that the speed that the motor turning the spool pulled the filament onto the spool would increase as the diameter of the spooled filament increased. This could be remedied simply by turning the spool with a wheel against the inside of the spool rather than the rim. As the filament accumulated on the spool, the wheel would run against the already spooled filament, changing the effective gear ratio as the diameter of the spooled filament increased. This would all mean that the speed that the spool turned would be constant in terms of millimeters per minute of plastic being wrapped around the spool rather than being constant in terms of degrees per minute of rotation.

This would probably be easiest to implement using a motor with a right-angle gearbox. You could even get one with a rubber toy/robot wheel already on the shaft that could be used to turn the spool.

It would be cool to use this to spool the filament directly from the extruder without letting it pile on the dusty floor. It seems that it would be difficult to set the speed of the spooler to match the rate of extrusion accurately enough to not pull too much or fall too far behind over many hours.

A useful addition might be the use of limit switches or optical sensors that will switch the spooler on when the filament droops to a certain point, and switch it off when the filament rises a set distance.

Excellent work. Some questions: Is your power supply 12V or 24V?
What is the rpm of the 12V DC motor?
Also many of your ebay seller's item does not exist. So validating your document pricewise is a bit hard.

I would suggest something like:
24V 2A DC power supply
12V 300rpm (gear ratio 53:1) DC motor

Best,
arcol.hu

Again - excellent! Can't wait to build one!