They buy books. First off, for magic Content is King. Most magic books are rules books. They give tricks and presentations and patter, in an orderly fashion. A book may be praised for having 'useful illustrations' that better define what is happening, but rarely is the art commented on in an aesthetic sense.
Indexes are luxuries.
Sometimes, spell-checkers are luxuries. In many ways, this echoes the earlier days of RPGs, when a photocopies set of rules was 'a viable product'. In a similar fashion, items one step up from photocopied pamphlets are still standard items. Most other books are limited run small press. They have very high prices and small quantities.