Questions about my new used 24-70mm f2.8

It could be dried cleaning solution or something else! Its just difficult to  tell from the photo. Have you tried to reclean the rear glass, that should answer the question.While it makes sense to look at used gear on expensive lenses, there have been enough issues with the 24-70 f2.8 that I personally would not purchase an used one from a private party I did not know well.