Always know why are need a mobile and why are buying a mobile phone. Buy according to your own needs.
Always compare the mobile phone you are planning to buy on other platforms such as Amazon, Flipkart, other physical retail stores and then come to the decision from where you will buy the phone.
When you buy a smartphone, always open the box infront of the shopkeeper and check its belongings. Check for phone, charger, earphone( in case) , manaul, and warranty card signed by you and the shopkeeper.
Dont buy the phone on the same say you thought of buying a phone. Always research for atleast 4-5 days, then finally come to the conclusion. Never buy phone in hurry, otherwise you will regret.
My personal experience in buying mobile phone is that always compare, research and ask your well wishers for their opinion also. People who have purchased the same phone you are thinking of purchasing. this will give you an added experience.