There's a reason for that and it is scarcity. Some titles never get a reprint in hardback or at all even. That's before you dip into the realm of first or limited editions etc.I was unaware that this is how Oxfam conduct themselves. They're supposed to be an ethical organisation! It's a rarity for me to consider buying anything from them because their items are grossly overpriced - you can buy books brand new for the same price that Oxfam usually charge for used copies.
I held onto some of my university textbooks. The price some fetch goes into three figures these days. Others are worthless.
What I wrote above seems to be branch dependent. Some get a lot of donated books, others are scrambling for them. There's an element of sending unsold books to less busy branches but eventually space is needed and pulp fiction, especially, gets pulped or ends up on eBay via bulk sellers.