It's not Xur's fault, It's the Nine. Xur states "items belong to the Nine, not me" and "The Nine wish to speak to you" and "my function here is to trade" It seems legit as the Nine are 9 sentient super computers like Rasputin who are guarding the inner and outer worlds from harm. They have sent Xur to this final stronghold of the inner worlds to equip Guardians with the best possible weapons and armor to protect the inner and outer worlds just as the Nine are.