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meggie We Buy Books won't collect from my place in Norway. :-) When I tried to find out what they would offer for one of my books the web site insisted that I create an account and hand over my telephone number and address.
Anyway most of the books I'm getting rid of just now are either technical books or textbooks and old well used paperbacks that are undoubtedly valuable to someone but definitely don't have a large market.
I've just looked at https://www.amazon.co.uk/Engineering-Mathematics-Programmes-Problems-Stroud/dp/0333333373. A brand new copy of the latest edition is 29.14 GBP. Second hand paperback copies like mine are selling for between 2.79 GBP and 5.40 GBP. Not enough to make it worth the effort of finding one of those niche buyers. If a commercial seller is offering it for less than three pounds I can't see them paying more than a quid for it because it will probably sit in the inventory for months; it's already been advertised here for three months and no one wants it despite me listing it as free for the last six weeks just like the books I gave away this morning.
I own about four thousand books now, down from about five thousand a few years ago. I Imagine I paid the equivalent of somewhere between 5 000 GBP and 15 000 GBP at today's prices for them over the last fifty years (the majority were already second hand and many well used). I'd be extremely surprised if they'd fetch more than 500 GBP as a job lot. A few might count as minor collectibles merely because they are a century old but not enough to raise the total value by much and some are probably rare in the West but that is simply because they were published, printed, and bought in China not because they are globally rare.
So these days when I decide to get rid of a book I list the more interesting ones online in the hope of finding a good home for them and donate the more mass market ones to one of the free libraries. One day I will have to cope with the problem that some I regard as valuable are completely unwanted and will have to be recycled.