They are quoting amazing prices. For my 128GB 13 Pro Max Comparemymobile.com is showing best prices of just over £550 with about £10 difference between them (Vendi, WeBuyAnyPhone and MusicMagpie showing as the top 3) whereas ReBuy is giving a price of £652 which interesting is the same regardless of whether I declare the screen and the back as "Very Good" or "Like New" (they are both like new imo). I was a bit put off by the web site though, when I tried to look what prices they were offering for other stuff (Tablets, Audio, Wearables etc) every single one of those links went straight to a 404 error page. I also don't see any contact details. I suppose that might be there if I went further through the selling process but I do think they could improve their site to make it a bit more reassuring.
As a counterbalance though I looked them up in TrustPilot and that gives them a 4.8 "Excellent" rating based on 3,761 (at time of this post) reviews and also does list a UK address and email and telephone contact details. For £100 extra I'm definitely considering this option. Thanks for pointing it out.
For convenience here is the link to the TrustPilot page I found -
https://uk.trustpilot.com/review/re-buy.co.uk