Then you show up at your local Honda dealership to find they are only selling Toyotas. Thats why OP said its all luck, because sure you can earn coins but you cant earn good items in Xur's shop, they will be there or they wont, what you do has no effect on his shop. When you goto buy a car you know what they will have, you show up to a Honda dealership they will be selling all of that years model of car that are made by that company and possibly some from the last year depending what time of year you are there.