I agree with the above and you have to be very, very careful as Amazon and Ebay are rife with bootleg boxsets, especially if the price seems too good to be true. I bought TNG online for what seemed a reasonable price at the time; only for about a fourth of the episodes to be unplayable on a second viewing. I had the same issue with the B5 box set and with the DS9 series it started freezing and chunking up on the very first viewing of the very first episode and I had to send them back and get a refund from Ebay when the seller didn't respond.
One way to tell a bootleg, other than then it paralyzing your player, is the writing on the discs is misspelled saying something like "Starr Trekk" or similar. Unfortunately, you can't tell till it's too late and even if it can be proven to be genuine copies definitely don't expect a good deal as the Powers-That-Be have been gouging the fans for quite some time now on DVD box sets. Even if legit, used discs suffer the same issues as most previous owners don't exactly handle them with kid gloves.
To me, the technology of DVDs is too prone to fragility anyway and they wear out just as a matter of course even if there's no visible scratches or damage. I'll never buy a box set again and if I want to watch older shows I'll either do it on Netflix, Amazon Prime, rent the discs from the public library or just download them from a peer-to-peer site.