Well i as a sensible buyer first thing that i ask is if their are any defects or if any crashes,i did ask him  about the proper functioning of the gps he lied to me
His words were
"Last time i flew it i tried rtl failsafe and it did work
Compass calibration error will go away after doing advance calibration
Gps is working perfectly please don't worry about it "
Gps is not working at all
Naza goes into atti mode every time
And when connected to pc
Some times it shows calibration error contact dealer or it doesn't show gps at all