If its a $4 dvd, then I can 100% guarantee that they are selling below cost.
With items like video games, dvds etc its a totally different ball game.
Say if I buy 100 video games at $30 each, my total cost is $3000. Typical Video Game Mark up is 10% after eBay and Paypal. So I'm making $3 a game. I have to sell 91 games before I make my first $30, if I don't sell all 100 I lose.
So the $4 dvd, probably an old title. With DVD's you simply have to cash in while its newly released. Say I order 2000 units and mark them up again $3 each. I will sell 1950 units and make $5850, I then have 50 units left, they sit around for a few months and then hit the bargain bucket on eBay at $5 each. My cost price is $10, so I lose $5 on the last 50 copies and still make $5600 off that one title.
And that is how people do it...