I want to suggest the following system:
First Owner System/Signature:
An item marks the first owner to craft it, drop it or earn it via a legendary quest line.
Expertise Down Grading for Non-first owner items only:
Items you have equipped where you are not the first owner, will be downscaled to your expertise. This will prevent self-crafted and questline drop items being down graded for the person who achieved them, but still limit the effects of buying high level items on the Trade Post.