Personally I don’t see it as a problem when someone posts some art they did (commissioned or otherwise) that has D&D relevance, and in the body of the post mentions “I am taking commissions, DM me for details” or whatever. However, it becomes a problem when they’re posting everything they make, and it’s clear the intention is to fish for sales, not to show off some neat, topical art. It’s very hard to draw up hard and fast rules for this, and I think everyone would have a different idea of what’s ‘too much’.