Rent-a-Girl (1965)

A naive young woman looking for fun stumbles upon strange neighbors living in the same apartment building which she resides in. They start her off with a career in nude modeling, but in actuality turn out to be a ruthless brother and sister team who spearhead a kinky "girl renting" business for eccentric rich folk who get their jollies out of bondage and "whipping little girls 18 to 22".