They a Government Licensed organisation. Nearly like a Bank.
There Job is to make sure the transaction between you and sellTrade is completed – safely and securely!
This way you are guaranteed the payment for your iPhone because we pay them the money before you even send us the iPhone.
Your money for your iPhone gets secured by them and they act as a third party between you and us(sellTrade)
This way they reduce the risk of fraud by acting as a trusted third-party that collects, holds and only disburses funds/money when both Buyers and Sellers are totally satisfied.

Easy as that!