Most of these questions are answered on other discussions -- there was at least one other pricing/mark-up discussion started today -- as well as other helpful links on the site.
How does the actual sale work? You put a price in and if someone buys it, you get paid your mark-up on the 15th of each month provided enough time has passed.
It's mark-ups here, not commission, althhough premium members do get a small percentage above their mark up for costs of framing, etc.
How much more should I mark up my prints? One of the most asked and answered questions here. You should find several similiar artists to your work and look at their pricing. Show Price Details will give you a ball park figure of their mark up.)
Can I go back in an make changes to the mark up amounts? Yes, there's a button called "Edit" under each image in your portfolio.
Do I have to sign up for the premium package in order to sale my prints? No, but the limit is 25 whereas premium members get all they can load.