Natalie Hall, 16, of Ontario, Canada, said she learned the hard way that you have to wipe your phone completely clean before trading it in for money.
"'Why not?'" she told Inside Edition she thought. "'It's worthless to me.'"
She handed in her old phone and got $11, assuming it would just be recycled. But six months later, the phone was repurchased and she began receiving creepy messages, she said.
"Hi natalie ... All data is in the phone your so sweet," one of the messages read.
Natalie said she couldn't get the man to go away. "I had pictures of me and my friends in bathing suits [on the old device]," she said. "He kept trying to follow me on Instagram."
So what can you do to avoid what happened to Natalie? Inside Edition spoke to Steve Hruska, an engineer at the data recovery company Ontrack, who said it's important to remember that discarded phones still contain sensitive personal information.
"[It's] a gold mine for someone looking to do some damage with personal data," he said.
Hruska showed Inside Edition some of the things he's found on old phones, including private photos, medical records and banking information.
"There's some naughty photos people take they don't want on the internet," he said.
Hruska recommends wiping your old phone clean before trading it in. On an iPhone or Android phone, all you have to do is go to settings and scroll down and choose "Erase All Content."
If you can't figure out how to wipe your phone, never trade it in. Instead, Hruska said, send it out to be shredded.
"Absolutely have it destroyed," he said.
As for Natalie, she said she's blocked the creep who was bothering her and learned the importance of protecting her personal data.