Though this is a very late reply, hopefully those thinking of trying used BD at Amazon may read this as reference.
Occasionally, I bought used BD movies from the secondary market or third party sellers. So far from my experience, all the discs work perfectly i.e. they can playback well without any problem. Discs condition was also good. As for the BD casing, I'd say 95% are in good condition. In fact most of them are in very good condition (most were just watched once only). The other random 5% may suffer minor scratch, normal wear & tear, etc. Nothing serious.
If you're planning to buy, get it from those reputable sellers with good high rating. Actually, it's a big saving buying used BD as compare to getting new BD. From time to time you may able to find some bargains. Some sellers even offer free shipping.
Check out this seller who usually offer free shipping:
http://www.amazon.com/s/ref=nb_sb_noss?url=me%3DAM8SZZ9MYFOPH&field-keywords=blu-ray