I was wondering what happens when I sell my P3P and someone else will be using it. WIll the flight log be still conected to my account? Should I reset something before selling drone?

You could copy the pictures and videos from SD card to your PC, then format it.
When someone buy it, he will register his own account, then all the logs of his flights will be associated with his account, and logs will not connected to your account any more.