OP was asking about buying from Xur. If you do Weekly Heroic at 30 you're guaranteed 9 Strange Coin. If you take those coins to Xur and press the button on No Land Beyond, you're guaranteed a No Land Beyond. The only thing "lucky" about that is whether or not Xur brings something you want. You've earned the currency, so by extension you earned whatever you spend the currency on, since currency is just a placeholder for earnings.