I don't get it either. I do a lot of patroling. To me it seems like about half of the stuff I have to collect from the enemies goes straight to Eva Levante. But when I'm back at the tower and check her shop she sells the same stuff. I demand an explanation. Where do all the materials go that I collected for her patroling countless hours. I suspect Xur is the head of some sort of Tower Mafia. This is the reason why he's only around for the weekend. I bet he just checks in to collect his weekly danegeld. All the exotics are just to blind us so we don't see the truth. Xur sells the best stuff but is likably directly responsable for the fact that every single other shop sells nothing but crap. Think about it guys...