I don’t think the external sign of wear are any indication of the quality of a used lens.
For example, if you bought my 85 1.2L you would be buying a lens that looked VERY used, but one that was awesome in terms of sharpness.
With the high resolution of the current crop of DSLR’s there’s no latitude for mistakes in lens manufacture and until very recently a lot of stuff from the big players has been caught out as sub standard.
As an example, I tried 3 Canon 50mm 1.2 L’s before I found one I thought was up to code.
Nikon had the same problem for a long time as well.
Everything I’ve bought new in the last year has been 100% so I assume they have the message.
So, there would be a lot of stuff on the second hand market that might have been rejected by pro’s and you should IMHO have a VERY close and detailed look at pictures from any lens you’re thinking of buying before laying out any money.
Try before you buy.. 😉
www.dsaphoto.com
A picture is thousand words that takes less than a second while a thousand words is a picture that takes a month.