Android Phone Data Problem
It turns out that when you erase the data on your Android phone, they are still there.
Then how can you get rid of the data? You can't.
"This can be desperately complicated," said Ross Anderson, a Cambridge engineering professor who worked on the study.
Per Thorsheim, a cybersecurity expert in Norway, offered a different, more brutal approach. "Don't hand off your old phone. Smash it," he advised.
//www.garynorth.com/snip/1188.htm
Conclusion: do not sell your Android phone.
[Note: there is a multi-step procedure that can work. Take it or smash it.]