I don't think that there are any set rules for trader status, however I would think that admin would consider each application for trader status on it's individual merits. E.g. a new member who is know to admin or mods, proven history on the forum, an member offering professional services (e.g. a car mechianic with their own business) or a site sponser.
Infact I am surprised that there are not more members to this group (like the guys who do the body lifts & coil spacers), although these members may have been approached by admin, but not wanted the status.
Having this new group may well assist in recognising members who it should be safe to trade with, rather than the spammers who post items for sale, take your money, then bu99er off, while keeping the site admin out of the loop and not responsible for items bought or sold on here, as per the forum rules
Traffic lights in London are just rough guidelines