Every year I upgrade, I sell my 'old' phone, which is always in great working order with no cosmetic issues. Until last year, I've always used Envirofone.
Last year, Music Magpie were offering £50 more and I took the plunge with my iPhone X. HUGE mistake. They claimed it was 'scratched up' and reduced the amount by over £200. I asked them to send it back and thankfully, Envirofone honoured their original price for me even though Music Magpie took over a week to return my phone (which wasn't 'scratched up' at all).
Unsurprisingly, I'll be using Envirofone again this year and can't say strongly enough how much I'd avoid the jokers at Music Magpie.