If I'm not mistaken you pay some fees to them whenever you sell something. It was something like 10-15% I think...
If you want to sell you have to make some sort of a seller account on their website which costs some money but grants you various advantages.
The main amazon scheme is sending your merchandise into their warehouses so that you can use the Prime service. This way you just have to buy the merchandise and send it to them (along with a couple of other bureaucracy-related stuff) and they take care of everything else, storage, packaging, shipping, returns, etc.