So you're annoyed that someone (who took the time to go to a charity shop, list the book online, and ship it to you) charged you the RRP for the book, that you didn't have to buy from them?

I hope you have the same kind of energy for when you pay mega-corporations anything from tens to thousands of pounds for products that often cost single pounds or even pennies to manufacture (due to underpaying for labour and materials that were in turn manufactured by underpaid labour as well).

The person you bought this from likely works for themsleves, trolling charity shops all day for bargains, and almost certainly pays tax on their income. I'm as anti-capitalist as they get, but even I can't take issue with this. If they had charged you more than the RRP, sure, that'd pushing it, but if you didn't want to pay full price, you should have spent your own time looking for the bargain. ¯\(ツ)/¯