We get most of our media new from places like Amazon, etc and probably about 90% of the media we buy on eBay is brand new and sealed as well. There are just some things that are now out of print and if you want them you must buy used. Being on eBay for over twenty years we have pretty much learned what to look out for. The key for us is to buy from those selling off parts of their personal collections or established individual sellers with excellent reputations that they want to retain.
We now totally avoid buying from large sellers who appear to be basically online pawn shops. They often do hundreds or even thousands of transactions a month and some do not check their merchandise very carefully. There is one group that does business on there that we bought an item from quite a while back and will never do business with again. They brag about how the items they receive are all meticulously inspected and only the highest quality ones are listed which is total bull$hit. The particular item we ordered was described as like new condition but used stock pictures to show it. We knew better but the price was great and we foolishly bit. It arrived with wear and fraying on every corner, divots in the cover, front was actually sticky and had a piece of dried food stuck to it and the discs were scuffed and scratched.
Reported them and got our money back - lesson learned. Only buy items that show detailed clear pictures of the actual item for sale so you know exactly what you are getting. You can't avoid every shady deal out there but, in general, our track record is very good.