Part of the problem also is 'booksellers' who do not know a rare book from a bag of hammers, nor do they have any idea on how to price a book at a level which is high enough to give them a fair profit, and low enough to actually sell. They may also have no experience within a particular field, and may have no idea if a book is genuinely rare and sought after or just rare and unwanted. Furthermore, they may have no idea of any track record of sales of a particular title, and so they are just fishing.
Case in point - Chumbley's AZOETIA. The only copy on Amazon is a paperback priced at about $1300 - which isn't really too far out for this book, although the condition is listed as 'used - good', which I would translate as 'poor copy, overpriced and buyer beware' so $1300 is likely a rip-off for a $500 copy. BUT, on Abebooks.com, the single copy available is a Sethos edition green cloth hardback, priced at £ 1762.50, or circa $2800, which is completely bonkers - it generally trades hands at $500-$800, which is up considerably from the $300 that it was at just a few years back. I take this price to reflect general enthusiasm and a lack of knowledge, rather than an attempt at a blatent rip-off.
As a bookseller I know that there is a fine line between a fair price and a non-fair price, and that it is entirely subjective; However, a truly fair price is one in which both buyer and seller both feel a bit ripped off - if the seller thinks he has sold too low, and the buyer feels that he has paid too much, you've likely figured out the fair market value for a particular item.
I could go on, but I will spare you and I won't!