It's Marginal Revolution that specializes in "markets in everything." But I want to join in.
I was just quizzing the guy wandering Evans Hall buying "excess" textbooks from professors. He ships them to Barnes and Noble in Missouri, which then distributes them as "used" to college bookstores. He does about 3000 books a year, he says--working six hours a day forty weeks a year.
The college bookstores sell the textbooks at $65 each. They have next to no value sitting unused in my office. That's $65 x $3000 = $195,000 of value created a year, of which (if the $25 a book I get is typical) $75,000 goes to professors, roughly $15,000 to shippers, leaving $100,000 to be split between him and Barnes and Noble--plus the consumer surplus going to the students.
A complete welfare analysis would have to take account as well of the effects on the new book market, with its large pockets of local monopoly power once a book has been adopted for a course. But I don't have any more time before my next committee meeting.