That's a difficult one to answer. Anything below Ex+ may have marks on the glass. That being said, I bought an old manual focus 50mm f1.2 from them for the use of reverse mounting for some super macro stuff that had no marks on the glass at all, but you won't know that until you receive it.
Personally, I draw the line at glass flaws because I don't want anything that may affect image quality. Yes, I know that small flaws don't usually make a difference in image quality, but they can cause flare to occur where a pristine front element will not. But, if you never shoot into the sun maybe it doesn't matter. Like I, said this is a difficult one to answer as it depends on your lens buying preferences and shooting styles.
I recommend taking a look at KEH's return policy. If I remember correctly, they allow returns in the same condition as shipped for something like a 14 day period and you're only out shipping in both directions. If the shipping isn't too expensive, you could try one of the lower rated copies and return it if you have any problems after giving it a good workout.