I have many old glasses and that's what I check while buying
#compatibility :as i am into nikon system hence that's not much of a problem unless you are going for pre Ai era. But for other users, for instance Canon, fd lenses will not focus to infinity on ef/efs bodies. So take your camera body along and test first hand.
#give the lens a shake to ensure that no moving parts of loose elements are there.
#check that no fungus / haze etc are there.
#minutely check front and back elements for loss of coating.
#shoot at different f stops to ensure that the aperture is not stuck or playing heavy.
#rotate the focusing ring so that to ensure that it is smooth.
#check for body dents, taps etc. Carry a filter with you and try to attach it. Many old lenses have bent filter rings and that's a real pain.
#don't buy distance / online unless the seller is known ( I have found out the hard way).
#If an AF lens , check for AF accuracy wide open and stopped down both. Carry a laptop to check that , don't rely on LCD.
#If a VR lens check if that works.
#Check the condition of the switches / selectors and see if they are loose/wobbly/stuck
Last but not the least, check the images not on camera lcd but on a laptop screen. If the images come out well, even if there are fungus, dents etc you may go for the glass and most probably you will get a good one at a great price . Most of lenses will have some dust inside so that doesn't matter much. But don't buy lens with fog or haze. Have I covered everything?
