I'm in two minds about buying an iphone 6 off instagram?

I'm in two minds about buying an iphone 6 off instagram. The phone is cheaper than normal price, however the person providing the phone bulk buys so therefore the phones are cheaper. Also, the phones come with 1 year apple warrante and can be returned if not happy with the product. The seller also shares customer reviews and feedback. So i was wondering and needed advice on whether i should buy the phone?
ps- the person claims they are a registered seller, what does this mean?

Being a registered seller means that they have a license to retail Apple iPhones

Deals that normally come through social media are often dangerous because you can always meet dodgy guys out there, so here's a checklist you should use:

1.) Make sure that before you buy the phone, you at least do some background checking on them by checking their social media accounts. (Just to make sure he/she is not a creep)

2.) Also check if they have a physical store or office space because every licensed retailer should at least have a physical store space.

3.) before you buy the phone, ask them for a proof of order/receipt document for that specific phone. If they can't produce this, it may mean that the phone was stolen.

4.) When you meet to exchange the money for the phone, make sure you are not alone and you're in a semi-public setting.

(Continued in comments below).

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