Good day Erwyn,
You do not need any special permission to post on the forum after you have signed up, just make sure that you read & understand the rules as it will help you navigate the site a lot easier. The
Member Rules (usually found from the Forums button dropdown menu on the site header [Forums -> Member Rules])
For posting, please refer to the sub-forum guides found
HERE, these videos will again help navigate certain processes like making a post & uploading images for future reference in video format.
As for posting, navigate to the classifieds section by using the gray navbar found underneath the donation drives tile, from there you select the relevant sub-forum pertaining to your items you want to sell, and the when creating a post in that actual section a template is already available for you to fill out, just make sure to read the template fully before deleting anything in the template, as the guidelines for using the template is explained in the lines that start with a "#".
If you are unsure still or need some guidance, you are welcome to make another support ticket, reach out to one of the moderators, or asking the forum in one of the Discussions sections of the forum.
Welcome to Carbonite & happy CUD
