You can buy one light at lvl 3 and another at 5. From the looks of it there is no option for the lvl 5 light. This would imply once you buy one at 3 it will lock until lvl 5. It is only a theory until some one hits lvl 5 but the option will no longer be there after your lvl 5 purchase. Bungie would not let us farm glimmer and light that easily. Edit: per players that used the reset glitch this is confirmed. Two per character people.