Knowing what size bike to look for
While Michaels answer is the more correct one, I found ETT or effective top tube length quite useful for me when buying second hand. It is easier for the seller to measure, and was more useful to me than the seat post height since that is trivially adjustable and my upper body is smaller than it should be for my height.. 

Edit: The effective top tube length is the horizontal distance from the top tube/head tube junction to the seat tube. This measurement has become significant as more and more bikes are designed with sloping top tubes. (from https://www.bikecad.ca/effective_top_tube_length)