I am looking into the Canon 200mm f1.8 lens and of course all of them are used since they quit making them. I noticed on ebay that sellers like to tout high serial numbers. Is that just to say they are newer or is it known that later models had improved optics?
I recommend you stay with negative numbers, the provide the best value. I have also heard that real numbers are to be preferred to complex (which might indicate feature creep and a potential failure).