Magic relics are fraught with dangerous magic traps, and guarded by deadly extraordinary creatures. Not to mention lost sub-dimensional planes, the randomness of opening and closing their entrances alone is enough to deter most wizards. No one knows whether the entrance to the sub-dimensional plane will close immediately after they enter, and it may not open again for decades or even centuries.
The act of Noland Lee's knowledge trade, to a large extent, is similar to directly placing the treasures of magic relics and sub-dimensional planes in front of Mabel.
Using magic relics and sub-dimensional planes as the sources of unknown knowledge is enough to launder these pieces of knowledge.
This is the common knowledge in the Realm of Wizards that Noland Lee learned from Denise Green. Denise herself never did this, but during her stay in the Ulnas Circle, she had heard about many such deeds.
