I completely agree that there is no problem, in general, with the idea of buying consent. It's what happens in a million transactions everyday. Every time someone agrees to provide their labour in return for a wage or to sell a product in return for its price, the employer or shopper who purchases the labour or product has bought the provider's consent for them to have it.
In fact, I've never seen the phrase "you can't buy consent" before, though I don't doubt it is going the rounds. When it comes to things like prostitution, the argument is usually a different one, namely that just some commercial transactions (i.e. just some purchases of consent) are off limits because they are cases where consent is invalid or inauthentic
The clearest cases occur when there is such an imbalance of power, such a vulnerability on the part of the 'consenting' person, that they are, in effect, compelled, and the consent is purely nominal. This is the case of many, though obviously not all, sex workers. Especially trafficked sex workers.
There are other sorts of cases where people might believe that it is simply not ethical to consent to certain things in return for money (even if there isn't an imbalance of power). I think I recall correctly that there was a legal case a few decades ago in which members of some sex ring or other were agreeing to various types of extreme mutilation in order to satisfy their own sexual desires and those of the people mutilating them.
The court decided that these mutilations were the sort of things that you just cannot consent to, and that therefore the mutilators were guilty of criminal assault. I can't remember whether money changed hands in this sex ring. But the legal (and possible the moral) principle was that consent to such things is never valid , regardless of any financial transaction.
So, tl:dr: the slogan "you can't buy consent" is false, but that is a complete red herring, because all that needs to be shown in relation to discussions around sex work and similar is that there are some situations in which consent in return for money is invalid.