The newspapers report "the Mann Act has been used...in prostitution cases, although normally it was the purveyors of prostitutes, and not their procurers..."

That seems to set them up as seller and buyer respectively.

On the other hand, in "A Funny Thing Happened on the Way to the Forum, I seem to recall an exchange:

Senex: "Senex, Pro-consul."
Pseudolus: "Pseudolus, Pro-curer."

I have always understood that a procurer in this sense was someone who procures customers for prostitutes (a pimp or ponce). I don't see that a purveyor of prostitutes is any different.