People like Vitesse are to blame, I doubt if much escapes his eagle eye for the charity he assists, but I have found seriously under priced stuff on ebay. If they run to the end unnoticed, they sometimes go for nowhere near what some specialist shops are pricing them at on AbeBooks, I've just bought a mint unopened copy of Innes Ireland's All arms and Elbows for £9.50, but please don't tell anyone.
As regards motor sport/transport books we get very few donated. Pricing for most of our books is determined by two other volunteers, although I do give them help if required. I also individually price local history stuff, which is often quite rare, even if recent.
My policy when putting them on eBay is always to put them up at a competitive price. And I do always bring motor sport and similar to your attention ...
I'd rather have the money in our Paypal account than the book sitting on the shelf!
My colleague who runs the charity's specialist bookshop in Bath, OTOH ... If I had the run of his shelves I could double or even treble my eBay sales without affecting his turnover! But as my Area Manager puts it, "he's a bit precious about his stock."
My personal purchased bargains, BTW, were a mint 2000/2001 Autocourse for 50p (I think they'd mistaken it for one of those 'official F1 annuals' which were £3.99 in The Works) and a copy of Wheels Across Australia by Pedr Davis for £1. How that found its way to a charity shop in Midsomer Norton I'll never know!