Paying someone to give up their bodily integrity is coercion though. It’s why we (the UK anyway) don’t allow people to be paid for organ, egg, or blood donation, or surrogacy. Consent for anything to do with bodily integrity should always be given freely - a clear and unfettered conscious decision.
But then I suppose this depends on how much you value bodily integrity - and that’s possibly delving into philosophical grounds as much as moral.
Moral grounds are philosophical, always. I think any discussion like this gets into all kinds of things that go well beyond the question at hand.
Bodily integrity is an interesting idea for instance. What does it really mean?
I tend to agree that certain things simply should not be commoditized on principle - they are not things that are transactional, and I'd include sex there, giving blood etc, as well as some things that are from totally different categories. The way I would say it is that it goes against their nature to be bought and sold.
I also think that there is a practical argument that making some of those things into commodities would tend to create a marketplace where exploitation could easily occur, and that's connected to the nature of the item being discussed. In some cases maybe that can be mitigated.
But what I wouldn't say is that is about consent being impinged upon by payment. If that were the case, it would mean consent was also being impinged upon in other cases where we pay someone, and I don't think that's true. It's not that it doesn't matter that when we pay a guy to refinish our floor that his consent to do the job was undermined by his desire or need for the money, because it's not that big a deal to non-concentually refinish a floor. The difference is in the nature of the act.
That being said, I think the idea that anyone makes any choice fully freely is naive. Very often I suspect we don't have much real choice at all.