ronk said:"I find it buckles under load as the wheels allow the jack to move."
It should move as you jack the car up as the lifting arm swings thro an arc - in effect the lifting point move horizontally. That why removing the front wheels is not a paricularly good idea either !
+1 for the Warning : Trolley jacks must be used on a smooth level surface to allow the wheels to move, no wheels induces angles and loads it's not designed to safely handle, it's quite likely to either collapse or tip and shoot the jack out at an angle