I used to work in used bookstores, and a significant part of my job was researching the books that came in to determine the price range and a fair price. Since the common used book-dealing sites do not list “Sold” prices, the range you’re often working with is only those that are available. Knowing that, there are two scenarios that can play out to get a book into ridiculous(ly unjustified) prices:

1. There are some used book dealers who do nothing but take the highest price and double or triple it, on the assumption that someone with more money than sense will simply assume the most expensive copy = the best/cleanest/nicest copy and buy it.

2. A particular book, at some point, ends up being unavailable online through most of the major sources (Abe and Amazon for the scalpers; a few other sites for the diligent). Because it is not listed on any common site, and therefore has no benchmark price, the greedy/inexperienced dealer assumes it is a very rare book and thus charges an exorbitant price for it.

Sometimes 1 & 2 combine and echo each other into the stratosphere: 1. inexperienced dealer lists otherwise unavailable book for high price (say $100). 2. shady dealer doubles that price ($200). 3. book 1 somehow sells (at $100). 4. another shady dealer doubles the first shady dealer’s shady price ($400). Lather rinse repeat. 5. Other dealers think they have expensive, rare book, as the only listings are for $200 and $400, and list it for similarly ridiculous price, somewhat lower than the only/highest price ($300).

I’m guessing something like that, and the fact that we’re dealing with niche books related to a hobby that gets one accustomed to splashing out, is what contributed to the skyrocketing price of those guides.