My Lumia 520 have been make demo device..

[Happy Rdv]

My Lumia 520 have been make demo content device nd make many problems eg-not sync microsoft account,not add other account like google,yahoo etc,nd not reset phone.

 

[WanderingTraveler]

If it's a demo device, just like you suspect, there are two things you should be able to do with it:
>Return to seller
>Flash appropriate firmware

First option is safer option.