At that price they're not really making much money. The disk 2-3$ the print on the disk is maybe 1-2$ then you have 10% fees on eBay and 3% on PayPal. That's like 5$ at best. Add time, risk and the power used to burn the disk they probably have like 2-3$
That aside sometimes it's nice to have even a free pice of software on a disk. And there are times that you need it on a disk.
I ran into a point were I did not have a running system and needed an OS disk and bought a copy of cent OS on a cd from my phone.
Edit:
The shipping is free too. I don't see them making anything over a dollar.