In the words of a well known large format lens purveyor, lens defects generally affect the user more than the lens. As Erie's example showed, obvious defects can be invisible in the final print.
Two of my lenses have 3-4mm scratches near the center of the rear element with no apparent effect, except on the price. Hence I picked up a 240mm Sironar-S ($2300 new) that was in perfect condition except for the scratch for $500.
I also have a 19" Red Dot Artar with significant cleaning marks on the front element that I picked up quite cheap. It looks like someone cleaned the glass with steel wool. I haven't done a direct comparison to a another lens, but it seems fine. If contrast is reduced, I haven't noticed. Unscientific to be sure (maybe I just have learned to develop longer), but it illustrates the point. Beyond that, common sense prevails. As mentioned previously, the price should reflect the defect, and a good return policy is your friend.
Check out the image quality this damaged lens can produce.