To open a shop you must PM an admin asking to open a shop and you need to show some things you made, Admins will edit your shop with a admin approved message.
About shops, You must be able to give the shop owner a certain amount of points,
if the shop owner wants 500 points for a sig, you must be able to give it to the owner. if you do not they don't need to give you the sig, or, you could give a second price(like maybe 300).
Don't rush, or harass a shop owner either, if you do they don't need to finish your sig or avatar.