There was nothing wrong with that, though. Rean and Trifas were both in the Saint Realm, even if one was in the Initial Stage and the other was at the Peak Stage. In the end, the level of weapons they could use was the same, Golden Initial-level, Middle-level if they had one. More than that, and their cultivation wouldn't be enough to use it correctly. However, Rean's Black Star was as strong as a piece of Golden High-level equipment.
Of course, that wasn't all. Rean's Black Star was made with World Essence from the system, just like Roan's scythe. While the Special Edition Weapons he sold were barely at the High-level in power, the White Scythe and the Black Star were at the very peak of what a High-level weapon could do. All of that while still being Golden Initial-level equipment. Let alone the fact that they could gather elements on their own.