Amazon, at least in the US, takes 15% of the sale price, a flat $1 per item in addition, and a portion of the postage you pay as well. When selling a CD to customers here in the US (I do this lots at work), we recieve a $1.84 postage credit, and on international sales we get $4.95.
Amazon makes lots of money off their marketplace sellers! $2.35 per $9 CD, and post $ as well. And all they have to do is provide space on their site.