Personally, Ebay and even private sellers on Amazon would be the last place I would look for used Dvd's/BD's. Years ago I bought the Final Season of Dexter, what I got was a very good copy that the average person would not know was a fake. The packaging was off, insert slightly off center, and the colors a tad faded. The discs were pressed. Thanks to AnyDVD I was able to detect it was a fake as all the discs lacked any encryption. I had the previous seasons to compare with.
Pawn shops are my place of choice and you never know what might show up. I found an OPPO 983 for $10 and a Sony bdp-s5100 RegionFree for $50. I have also found many OOP items (most notable was a rare IronMan Steelbook BD for $5).
Video store close outs. Second hand stores. Garage Sales. Value Village - I have found quite a few Logitech Harmonys for $3.
The forums at Blu-ray.com have a wealth of information for deals both used and new.