I've no doubt that the idea is already out there, but here's some suggested detail...
The Statistics shall contain both experience and "craft" or "label" points, which would be the measurement scale for artisans.
2 Options how this could work:
Option 1: When a crafted item becomes bound to a character (worn for the first time), the one crafting it receives Craft points equal to that character's experience (probably divided by some number, to keep these points on a similar scale to experience). When that item is sold, those points are lost, though perhaps some amount remains (say 10%).
Option 2: Every time a character (including mercs) earns experience while wearing one of your items, you receive the same amount in Label points. If they're wearing two, you get x2, and so on.
Better items would be worn longer and by higher-experience players. This allows the Label ladder to be an effective representation of an artisan's worth (which would probably boost their sales too, as people would possibly order items from them especially).
Hm, that last point gave me an idea - a section of the market where people put in commissions - For instance, I ask for a leather shield (rare) and offer 300 gold for it... The first one to comply with the commission would get the money.