While on this topic, this is a general opinion on buying second-hand railway books online, after buying dozens of books per year over many years...
I find World of Books to be probably my favourite of the big second-hand online sellers, and they have got better at grading the books over the years I've used them. I usually buy through Ebay (which is good for comparing prices from different sellers) although their books may be slightly cheaper through World of Books own website. Considering the volume of books they sell and the low-cost model of "pile 'em high and sell 'em cheap" they are reasonably consistent in terms of the grading for books listed in "very good" condition. If I bought ten "very good" books there might be one where I'm disappointed by the condition, but there might be a couple that look new or are real bargains. The plastic packaging is surprisingly tough and by keeping the pages pressed tightly together is usually enough to prevent damage; if a book does get damaged in transit you can usually see where the packet has been scuffed or knocked by a heavy object.
Another good large second-hand book seller on Ebay is "WeBuyBooks" based in Lancashire somewhere. Even if the condition is listed as "good" it will quite often be as good or better as books listed as "very good" by other sellers.
AwesomeBooks (Wrap group, Berkshire) are also usually fairly reliable when grading the books, maybe not quite as good as World of Books but not far off. They also trade as "Baham-Books" and list scarcer books under "Infinite Books." "Cmedia Group" is also the same company, I think they use that name to list books where the condition is not so great. I have a feeling World of Books used to do something similar where a lot of the books in "good" condition were listed under a separate name (something generic like countryside books) and the better ones sold under the World of Books name.
Other second-hand book sellers like Brit Books, Music Magpie, Greener Books, Goldstone Books and Cotswold Library are OK if the price is right, but are not my preferred option as they can be a bit more hit and miss. For example Music Magpie are not great at rating the condition of their books (I've had some listed as "very good" despite being a bit dog-eared). But then again, I bought a copy of "Swindon Engineman" (published by Wild Swan) for £10 from them recently and it was absolutely pristine, so it's swings and roundabouts.
Better World Books usually sell ex-Library books (listed as such) but sometimes you get lucky and it will be a barely-used copy from a reference library or something.
Generally, if something is listed as "very good" on a site like Ebay it should be in nice clean condition and look presentable although not perfect (a bit of shelf or age wear, a small tear in an otherwise decent dust jacket, a name written inside the cover, a minor bump). Books in "good" condition usually indicate more wear & tear but can also include things like highlighting, underlining or notes in the margins, missing dust covers, ex-library copies or just more general aging, grubby marks, slightly loose binding or other wear. Buying from individual sellers can be better if the condition is important (sometimes I want a nice copy, sometimes I'm not too bothered) as you can usually see some photos. I find buying from professional booksellers doesn't always give a better experience, as even if the packaging is better their definition of a grade like "near fine" may not quite match what I am expecting.
Professional booksellers (through ABE Books for example) probably make more sense when you are buying books which are scarcer than the average OPC or Ian Allan railway books which had large print runs. If I buy from somewhere like World of Books I have lower expectations but am often pleasantly surprised. I find buying from these companies through Ebay is usually cheaper than Amazon, probably due to selling fees, although Amazon does make a useful book catalogue for reference. Ebay also has a useful function where you can save searches and get an email notification when items matching it are listed - useful if you are after a book which is harder to find or the current prices are too high. A lot of books turn up for a decent price eventually if you are prepared to wait.
Another thing I've noticed is that occasionally a book will have the wrong image associated with the ISBN and the same error will copy itself over different selling platforms. Checking images from individual sellers can help clarify anything that seems like it might be wrong. The sellers I've mentioned don't take individual images of each book (except for Infinite Books I think) so they will be using a generic image of the book. Usually these companies list by ISBN - a couple of times recently I've ordered based on the ISBN being correct, and got the book I wanted despite it being described as a different book or listed with the wrong image.
