Which mobile should we buy under $300 budget?

The Elephone Vowney smartphone seems like the phone for you, I have not physically came across the phone as of now but the phone's build quality seems good enough for the price and the features it is offering.
If you are an innovator and could slightly increase your budget then you can get an eye sensor also.
Yes! An eye sensor, along with all the other features you are getting in the elephone. The phone is called "UMI IRON Pro", check the phone out and see it for yourself.
Anyway, if you're planning to sell your old phone or if you wish to buy an used smartphone or iPhone, I have something just for you, Gadzet.in. These people have revolutionized the way of buying and selling used smartphone online.

Ps: I am a representative of gadzet.in, you can rely on us.