AMA with Professional Classicist with a 20,000 Book Library (youtube.com)
In the early days of online books selling, bookstores would just put their books on sites and put whatever price they had been asking for in the store. This meant book collectors could get rare books for cheap, like paying under $15 for something that might be worth $1000. But from 2006/2007 things went worse for book buyers. Rather than using their own knowledge, built up over decades, sellers would just look for what other sellers were listing the price as, and modify their price based on book condition.
The peak of difficulty for buyers was 2012. Since then, there are 3 big online book sellers in the UK, and they have stock of 1-2m books each, they don't have staff to look at each book and appraise its value, and the staff they have don't usually know a book's true value. So he regularly now finds books for $10 which are worth $50.