Definitely not. The only way he could would be if each account, not character, could only buy one per week. And even then it is not preferable. Players should not be able to reach max levels without doing endgame activities. Xur is not endgame, anybody between level 20-30 could just save up some coins and skip up to 34 without ever completing a single iron banner, raid, PoE or nightfall if he sold etheric light. Max level should feel rewarding, not handed out. If Xur sold etheric light, you might as well have let the vanguards sell lvl 30 gear in the beginning for a few marks. No, keep prison of elders as the main source. Keep it a semi-rare drop as well. Make sure people truly work for it. Nightfalls, IB and trials should drop them on very rare occasions. But xur should not sell them.