The development worries some advocates working to prevent the sexual exploitation of women and children, who fear Backpage’s new payment system will make it easier to sell vulnerable girls online.

“It’s a message to traffickers: ‘Hey, we’ll protect your privacy,’” said Caitlin McCartney, a staff attorney at Legal Momentum, a national nonprofit that’s suing Backpage on behalf of a girl the group says was raped and sold five times after traffickers advertised her on Backpage.

“Not everybody has bitcoin,” McCartney said, “but everybody has pretty easy access to gift cards.”

Neither Backpage nor its lawyers responded to requests for comment.