I have had a mixed bag with Music Magpie. When upgrading to my XS Max I went through the process of selling my X - MM quoted me over £600 for it - going rate was about £485 at the time, not sure what happened but there website was obviously broken. I sent both phones off, immaculate screen and surround as they had been in a apple case, the only section which was slightly scuffed (as I say the tiniest of hairline scratches) was the bottom near the charger. Music magpie claimed there was dust under the screen, scratches and allsorts wrong with it. I received it back (in the same condition I sent it in) a few days later. I assumed they lied because they didnt want to honour the overpriced amount they offered. On another occasion I sent off my Apple Watch 3 with no problems at all.
When I got this phone back I took it to CPW, after spending 10 minutes discussing how great a condition the phones were in, him saying that they are the best he has ever seen, going so far as to asking if we actually used them, he then took them to be checked on the computer, then came back and offered me like £200 for them due to hairline scratches - I am being serious when I say they were invisible unless the light caught them right. I then went to another CPW store who checked them no problems and gave us the £800 or so (for two).
Ultimately I pooped myself for a couple of days thinking that MM were going to send me back a dodgy phone, but they didnt.