There seem to be a lot of devices like
on eBay. ( Search for Doppler Radar sensor ). Prices in the 7-12 USD range.
Anyone used one of these gadgets ?
Those are the radar modules without comparators.
It is useless.... until you build a radar detection circuit with comparators.
You should use the radar modules with a sensitivity pot on the back side. Those have a digital output if motion is detected.