OAKLAND, Calif. — Since she was 14, Tiffany says, she has been sold for sex, offered via hundreds of advertisements on Backpage.com, a website that grew rich on classified ads for services like escorts, body rubs and exotic dancers. Far from being a marketplace for consensual exchanges, Backpage, the authorities said, often used teasers like “Amber Alert” and “Lolita” to signal that children were for sale.

In the midst of a Senate investigation, a federal grand jury inquiry in Arizona, two federal lawsuits and criminal charges in California accusing Backpage’s operators of pimping children, the website abruptly bowed to pressure in January and replaced its sex ads with the word “Censored” in red.

Even so, Tiffany — a street name — did not stop using the site, she said. Instead, her ads moved to Backpage’s dating section. “New in town,” read a recent one, using words that have become code for selling sex. “Looking for someone to hang out with.” Other recent dating ads listed one female as “100% young” and suggested that “oh daddy can i be your candy.”

In the fight against child sex trafficking, shutting down an epicenter like Backpage was a major victory, but one against a relentless foe that quickly unfurled new tentacles. The demise of Backpage’s adult ads undermined the trade, but it also illustrated how difficult it is to stamp out the practice of selling children for sex. The crime is rarely punished with the full force of the law — charges like rape or statutory rape — officials say; in many places it leads to just a citation, instead of an arrest.