John Cannon recently closed Alladin books. That is a shame. However, Richard Hatch and Byron Walker are both very realistic about their prices.
Clay is right about non-magic book dealers. They have absolutely no clue what various magic books are worth. Also, they have no idea what books are scarce, rare or actually unobtainable. I prefer the term "unobtainable" to "out of print," because a book can easily be out of print and not be unobtainable.
There are a number of eBay sellers who often, if not consistently, list books as being rare, scarce or out of print when this is not the case at all.
Sometimes, they don't know any better. Other times, I know that they know better, but they don't change their listings when their errors are pointed out.