I'm so far removed from the attitude of "I will put this for free online then when it's popular put it behind a pay wall" I have no idea how the AO3 should deal with it, but it does seem against the general principles of the archive.
Commercial spam (including fandom-y commercial spam) is already prohibited by the TOS. If someone takes down their work before selling it elsewhere or posts a comic but also sells paper copies on their homepage or something, I don't think that would conflict with the TOS. But in either case, they wouldn't be able to use the AO3 to advertise.
Re: Ah, I think we have some unspoken assumptions in conflict
Commercial spam (including fandom-y commercial spam) is already prohibited by the TOS. If someone takes down their work before selling it elsewhere or posts a comic but also sells paper copies on their homepage or something, I don't think that would conflict with the TOS. But in either case, they wouldn't be able to use the AO3 to advertise.