I got £450ish for a slightly upgraded 2014 Boardman Team Carbon a year ago on eBay. People go a bit mad on there.
Old Di2, as in one of the examples, is problematic. 10 speed Di2 spares are like hens' teeth (a good USED rear mech is £500) and converting the whole bike to later Di2 spec would cost thousands. I don't know what SRAM parts availability is like.
I think a lot's changed in a year. I've long-toyed with the idea of selling my 2017 Team Carbon, and up until fairly recently they were holding pretty well at around £400-450; now they seem to be a lot less..
That's interesting about Di2, but sadly not surprising. Confirms one of my many reasons for staying well away from electronic groupsets..