Nick. One thing I've learned is not to be too fussy about lenses!!!

I was one of these people who verged on paranoia when checking both new and used lenses!! I agree about checking for signs that the lens has been dropped etc, but small marks (often from over zealous cleaning) and those small flecks/specks that cling to internal surfaces of the elements make NO DIFFERENCE whatsoever to the final image. BUT they do offer a bargaining point with dealers/sellers!

One good way of examining a lens is to open the shutter and hold the lens to the light and have a good look. If you find any of those pesky flecks, pull a face and sigh! and then offer a lower price!!

Regards Paul.