I've search Trademe completed listings but couldn't get enough hits to be able to get a decent ball park figure for a market price.
We've just renewed our carrier contract with Vodafone at work which includes a complete replacement of all existing handsets. I have a new iPhone 11 incoming shortly to replace my iPhone 8 which is less than 6 months old as it was a replacement for a unit that had developed a fault. So, less than 6 months old, battery health still at 98% and always in a case with no blemishes. I have the opportunity to buy it from work for $400 which doesn't seem too bad. Would be an ideal phone for my Father in Law who is recently retired and just handed back his company supplied iPhone 6. He doesn't need the latest and greatest but this seems the cheapest way to keep him in the iOS ecosystem which he is familiar with.
$400 seem about right?

