I'm on the side that a commission feature would best be left to off-side art sites. As an artist for commission myself, it's troublesome enough to contend with orders on multiple storefronts, and roll20 is first and foremost a site for playing games rather than trying to sell artwork. In consideration that Roll20 would also have to handle payments as a middle-man (in addition to whatever payment services may be involved) I can only see it being troublesome if a dispute has to get handled. If I have to contend with disputes, it would be best in favor of an artist's own terms of services than whatever likely royalties and rights need to be pledged for using Roll20 as a third party middleman for a market that is largely a client-seller business. This also coincides with the fact roll20 doesn't offer a payment front for paid campaigns, so I do not see the case being any different for artists. If anything, at most, I feel as though this problem would be best solved by making a Looking for Artists section of the forums rather than making a full-on feature.