Hello,

The fuel pump is attached to the moveable (up/down) fuel level sender tube. The small spring on top of the sender tube ensures that the tube (plus the attached pump) is extended down to the bottom of fuel tank. As long as the sender tube can extend fully down (helped by the spring) the whole unit is ok.

Once installed the sender tube (plus the attached pump) stays at the bottom of the fuel tank. It doesn't ride up/down within the tank.

The item that flops up/down within the sender tube is the float that sends fuel level back to the dashboard meter. It is freely moving. This is normal. Back in the tank it just floats according to the fuel level.

Regards,
Amarin.

EDIT: I concur with Oldyellr that the spring pressure is not critical. From the video I would pick the eBay item as you were having fuel level issue with your old sender unit.