It always sucks to know you paid more than the seller did - but that just means Oxfam undervalued the book.
Having worked in one, charity shops tend to have a habit of either really undervaluing or overvaluing their donated goods - cause the people who actually set the prices mostly just guess based on looks and nothing more. Only if an item looks expensive will they do any research, and even then never really enough.
Whelp, we’re one step ever closer to terminators. Just gotta let Boston Dynamics cook now