If there’s a fee involved I would call that “commercial use” that is explicitly forbidden by our Terms of Service, so I would contact help@inat or flag the message in question and explain the situation.
If there’s not a fee involved, I would still be suspicious, especially if the organisms seem rare. There are plenty of legitimate reasons to want specimens, but most of the legitimate means of acquiring them require permits from local land and resource managers. So: maybe ethical, maybe not, depending on your ethics and the context.
Would you be willing to share what species this person was interested in buying? This touches on a larger issue of how we at iNat can know what species are threatened by disclosing the locations of their populations. We currently rely on external authorities that provide designations of “threat” or “rarity,” but these often don’t specifically address the question of what species are valuable enough to collect / harvest.