If you allow artists to display their work in various communities along with the ability to post links in their profiles, but you restrict actual posts to disallow self-promotion, it’s the best of both worlds, IMO.

In other words, if you can’t include self-promotion in your community posts, but everyone knows you have the links in your profile, it attracts less grifters and keeps the feed clean, while allowing anyone interested to contact a poster directly or ask them promotional questions via DMs.

That said, hosting a full-fledged marketplace is not a good idea, IMO. There are laws and banks involved, which mean lawyers and taxes, and volunteer management does not work for that. There are already marketplaces that do that well, and allowing artists to post their own links of choice in their profiles will let them steer actual business to other platforms, while keeping the fediverse for display, review, share and critique. My opinion, anyway.