garage door usually won't close - even trolley without door won't move much
You did not state the actual model number so I will not be able to answer with model specific information that would be much more detailed and specific to your situation. So you will not be able to do this blind folded.

Since your sensors are working .. good. Some movement of the door is not an indication they are working - that is an indication they are clear to start with. You could have a loose wire or sensor or circuit board inside the senor that when the Door Moves the vibration causes the beam to break! - been there done that.

Why do the lights on my garage door opener flash and my door will not close

How to align safety reversing sensors.

So if those sensors are Truly fine. 

Then check your drive gear, it has a set screw or some means of securing it to the drive shaft that screw may have backed out and or is loose. The gear may turn with no load (in other words no belt connected) but might spin free if there is a load - Remove the belt and you can try to grab the drive gear by hand and hold it from turning while the unit runs. Your motor appears to have a gear box and the belt pulley attached - this style pulley if I am not mistaken is a compression style - so it might not have a set screw but rather the way it is secured causes a compressing effect and the pulley grabs the shaft - in short tighten the pulley to the shaft (it is most likely loose) as I said previous Make / Model number / perhaps picture of the name plate .. might have yielded you detailed pictures and specifics to what you have and you might have had it fixed in less than a minute..