In a free market a seller has a responsibility to their customer, if they are negligent then they will lose.

What responsibility does a prostitute have to her client?
Prostitute that know's who her clent is

She knows her client is married
Does she go ahead with the transaction? Is it moral or immoral?
She might have a decease, that could affect her clients wife?
She might get pregnant?

Her client isn't married
All well?

Prostitute that doesn't know her client

Does the lack of knowledge absolve her from moral responsibility?
She might have a decease, that could affect her client's wife?
She might get pregnant?