Buying 2nd hand bike

For a good saddle height, while sitting on the saddle and the pedal at a 6 o'clock position, put the ball of your feet on the pedal and see if your legs is straight, if it is, then the height is fine (your legs shouldn't be 100% straight).

Actually it's easier to put your heel on the pedal at it's lowest point, with a straight leg this will ensure your leg is slightly bent when the ball of the foot is on the pedal. (same thing but explained better)

7/10 fixie skidders have their saddles too high.