Most "media-type items" are generally obtained at very low cost. To that end, a lot of the big retailers in the ebay space will buy items by the pound at pretty cheap cost from not-for-profits that get donated that stuff and just want it gone. Even beyond that, a lot of people are simply giving books, CDs, and DVDs away. A lot of times, the ones that can do this do it at volume which helps on packaging (often just plastic bags) and the like. But generally, what the big retailers are doing is depending on volume. They make maybe 5-25c per sale, but multiply by 25K or 50K, you start getting into some solid money.
That said, the major bulk retailers definitely are pulling down the prices on everything, but it's the market and what you got that dictates the price. When everyone in person is giving away books at almost nothing and ebay retailers are doing the same, all you can do is follow along and hope for the best. Mainly because the vast majority of the market looking at ebay are just looking for specific throw-aways and not keepsakes. I don't blame them as they can do that.
But them, and the ones that are indeed deliberately selling at a loss (got stories on that) definitely are making it near impossible for the rest of us to sell those things and make it worth more than throwing them in the waste basket (as I just may soon with this 1099-K garbage in force).
