Spirit of the game: "Players should play and learn to get the best weapons and achievements, we don`t want them to buy it".
Effects on the game of that Spirit: Crafters can`t craft any BiS weapon.
Community: "Hey, im SELLING content, drops and achievements even!"
Devs: "Ok then."
Crafters: "What the f-"

In short, either give crafters the ability to create BiS gear superior to raiding or punish as a baneable offense raid selling.