Despite their improved image of recent times, many people still have an aversion of stepping foot inside charity/Opp shops and would never consider buying anything, let alone books, from one of them.
Many genuine finds can be found in Opp shops, and this is a maxim most second hand and antique book dealers live by.
Some of my friends have found out-of-print books in Op shops for a tiny fraction of what they are worth in antique shops, one even found a complete set for $50 which was later appraised at $1,500.
If anyone watched The Antiques Roadshow in Australia they would have seen the episode where a woman brought in a vase she'd just purchased at her local Salvos store for $15.00 - it was valued for thousands of dollars, and then even more again because she had the matching twin at home !
Opp shops usually get the rejects, the too-good-for-the-tip-but-no one-wants-it stuff; and in amongst the coal are sometimes diamonds.
With the advent of many charities having ready access to the internet some are shrewd enough to recognise and research a valuable item and price it accordingly but their prices are still many times below that of a licensed second hand or antique book dealer.
Also with education departments changing the required text books every couple of years, 12 month old text books (sometimes with the CD ROM), usually worth upwards of $50 can be found for less than a caffe latte would cost you.
Spending a few minutes browsing the shelves at an Opp shop could net you one of those best sellers you've been meaning to read but didn't have the spare time/cash to buy ; a hard to find autobiography you've been searching for for years; the sequel to one of your favourites ; a little known title from your favourite author; an ancient, fly-specked book that has you roaring with laughter; an outdated history book that has a true spin on popular, but repeatedly mistaken, tales; the possibilities are endless.
And at the end of the day, your few dollars for printed entertainment go to a charity that helps those with weightier worries than entering an Opp shop.