Peter forwarded this link on a local charity's book market. They have some restrictions on which texts they will take for resale. The list, informal as it is, provides insight into what they are able to sell based on experience (and how the consumers purchasing the text perceive the half-life of knowledge in certain fields. These are the books they do not accept (I'm quite surprised at some items from fields I expected would develop more slowly - like physical education):
University texts more than five years old: Accounting, Business, Economics, Education, Management, Marketing, Psychology and Sociology
University Texts more than 8 years old: Anthropology, Agriculture, Archaeology, Biology, Chemistry, Dramatics, Human Ecology, Medicine, Nursing, Physical Education, Physics, Geography, Political Science, Writer's Guides, Zoology