It may be helpful to add the part about adding yourself to the marketplace group on the rules page, I was able to figure it out but I had to find a bunch of threads to see where it was done 

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Could you please add me to the marketplace user group as well? Looking to buy and sell some photography equipmentI've added you to the Marketplace user group, so you should be good. We'll have to give you one more approval after you make your first post, so be patient. It will soon appear.
You need 25 posts. Make 'em good ones, please.Could you please add me to the marketplace user group as well? Looking to buy and sell some photography equipment
Ok, thank you! Been a long time thread reader and finally made an account. Exploring the forums nowYou need 25 posts. Make 'em good ones, please.