People selling stuff from amazon for cheap?

I was on Snapchat keeping seeing these accounts with loads of reviews saying they will sell iPhones for like £200 bank transfer and they have proof and **** is it legit and how does it work?

That depends. If it's a shonky old iPhone 3, 4, 4s, 5 or 5c then the price is a rip-off.

If it's for an iPhone 7, 8 or X then it's a scam: nobody legitimate sells them that cheaply.

Get ripped-off or get scammed. Take your choice.

It it sounds too good to be true, chances are that it isn't.