Was my iPhone used before?

Resellers other than Apple Authorized resellers do not have any access to Apple OEM new products. By definition, any and every Apple product they sell is used, stolen, gray market, or fake. And the default one year warranty period begins on the date of first retail sale by Apple or an Apple Authorized reseller.

There really is no way to know the history of your device, how many owners its had or what has been done with it in the past. It's used and that's about all you can know about it prior to the time you got a hold of it.

If you restore it from the backup of a previous iPhone, statistics may reflect data from the old phone.

You can input your serial number here to check the warranty status: https://checkcoverage.apple.com/

The expiration date is 1 year from the date of original sale. If it doesn't match up, then something fishy is going on.

The warranty expires on April 2, 2017. Almost a year ago. That means it was first purchased on April 3, 2016. Those are the only dates that matter in determining if it was new or used when you got it. As you bought it in July 2016, that was 3 or 4 months after it was originally sold. What was it doing for those 3 or 4 months? You also said it was bought as a SIM-unlocked phone from a 3rd party retailer. 3rd party retailers do not have access to SIM-unlocked phones; they can only be purchased directly from Apple. So in addition to being a used phone, it is not a SIM-unlocked phone.

This is a technical support forum staffed by users. It is NOT the World Psychic Society.

Your phone is used. Absolutely. It doesn't matter what's on the cables or the charger, the warranty expiration is MONTHS before the one year anniversary of when you bought it. That means it was NOT NEW when you bought it. Nothing else matters. You did NOT buy it from Apple. You did NOT buy it from an authorized Apple reseller. You bought a gray market phone from a scam artist who told you it was new. There is a non-zero probability that the charger and cable that came with your phone are genuine Apple and were originally packaged with the phone you bought. I wouldn't bet lunch on it.

Cables have a faint legend on them that says it's designed by Apple in California and manufactured in China. The legend starts about 6 inches from the USB end. It is very hard to see; use a bright light and magnifying glass.

I've never seen a cube made in India, but it's possible.

Check the warranty status. Does it match up?

So you got a used phone. It was originally sold in the beginning of April of 2016.

That's not relevant. You have a used phone.