You only get charged if you sell a domain. If the domain is parked with Sedo then the commission is 10% if it is not parked with them it is $50 or 10% whichever is higher.
You can only push a domain to auction after an initial offer. Under certain circumstances you can ask your rep if you can put a quality domain on a 10 day auction either with or without reserve. Alternatively you can submit your domain for consideration of being placed into a premium auction, this can be done through the management console using the drop down menu on the bottom left of the page.