I think the guild will control the business
related to there field of expertise. If you want o make an item once in a while for you or for a friend, it is more a personal matter than business. If you start to do that for aliving, you are most likely try to have the benefit of teh job system, which is for a crafter, buying a workshop, and thus have the permit.
Also, take note that having a shop allow you to have a thread for you,
as it has been proposed here. Your only other option is to use the RDI Trading-Post thread, but your owe 5% of your sale from to the RDI, so if you want to do large amount of money (like by selling lots of full plate mail at 1000gp each....) you better have your own shop (and thus, you'll be a memeber of the guild).
But as I told earlier, paying the 500gp for a shop for the merchant and artisan, and that include the fee to join the guild, that is a suposition I am making which I think reasonnable, but nothing have been officially approved... anyway, for such thing, I am not sure it need a proposal.