No advertisement is ever appropriate, period. Advertisements should be banned. I don’t mind a ‘look what I made’ post, but when the post designed to convince me to give you money, I see an immediate conflict of interest that suggests advertisement rather than information. It’s hard to draw that line without knowing intention, so I don’t think those posts should be disallowed, but if your post asks me to click a link to a product so I can give you my money, I’m downvoting.