Another idea, given that this is the Cartographers "Guild", is to actually move towards a more guild-like structure. Those new to the craft would enter the Guild as novices. After they had made certain benchmarks (the criteria of which would need to be established by the Guild, rather than a reputation system), they would move on to becoming Journeymen, and, given enough time, talent and investment within the Guild, even Master Craftsmen.
Moreover, if a more distinctly guild-like structure were put into place there could be financial and professional advantages associated with attaining a higher rank. For instance, the Guild could put into place certain standards for placing and accepting commissions. Commission rates could be indexed to Guild rank as follows:
Novice Level Commissions: $0-$75
Journeymen Level Commissions: $76-$150
Master Craftsman Level Commissions: $151 +
Now of course, Guild members would be free to pursue and accept commissions at variable rates outside the Guild (at RPG sites for instance), but were this explicit structure put into place within the Guild, I believe it would have an overall a positive network affect. Such a system would (a) offer clients a more concrete and realistic set of expectations of what kind of quality cartography they can expect for their money; (b) this in turn would give aspiring artists a goal to strive for; (c) over time, bearing a Master Craftsman title might come to carry weight outside the Guild, opening up professional cartography opportunities with RPG publishing companies, or publishing houses dealing with fantasy literature etc.
These are just a few brief sketches of ideas really, but I think there is potential here to be explored.