In each marketplace section there is a pinned post with rules regarding that section. Selling & buying sections have no requirements. Currency exchange section requires minimum 250 posts & 3 months account age. For SE/Refunding services you must have at least 1 month on site and 50 posts. Additionally you may be required to create a service giveaway in order to get your thread approved.
If you wish to sell something,go to the section where you want to post it and read the stickies. Once you have an understanding of the rules post your thread following them.If you do something wrong you will be notified to fix it. You won't be penalized in any way.