Abebooks is owned by Amazon, so a lot of titles are 'dual-listed'. However Abebooks has always been the site used by most secondhand book dealers and many prefer to only list books there, rather than on Amazon itself:
https://www.abebooks.co.uk
Other than Amazon, the biggest rival to Abebooks has always been Alibris:
https://www.alibris.co.uk/books
Although MusicMagpie is primarily thought of as a site for trading CDs and DVDs, it's also popular with people buying and selling books:
https://www.musicmagpie.co.uk/store/books
. . . and Wolf63 would never forgive me if I forgot to mention that Oxfam sells secondhand books online too. (She devotes many, many hours to sorting and pricing books, postcards, etc for online sale by them):
https://onlineshop.oxfam.org.uk
(Click 'Books & Music' and then select your preferred genre)