The value of a magical treasure is at least a million spirit stones, and such treasures have highly distinctive personal characteristics. They are basically custom-made for individuals, unlike those mass-produced magic artifacts. Often these magical treasures are quite renowned because the ones who can use them are at least strong figures in the Secret Realm, possessing a Primordial Spirit. Such powerful individuals are naturally not obscure, and the weapons they use would not be without fame, often being well known.
Therefore, if a stolen magical treasure surfaces, it's easy to trace where it came from and whose hands crafted it.
An auction house without a certain strength would not dare to openly auction such items. If they offend some formidable character, the entire auction could be overturned in revenge.
"Ordinary magical treasures are not as good as what I want to sell, so you better be prepared mentally."
