I'm a freelance writer and yogi living in Tepoztlan, Mexico where I'm running www.casadelcorazon.mx with my girlfriend. You can keep up with my other writing on www.bollocks2thewellingtons.com/
There are a plethora of secondhand bookshops in Chiang Mai and they all offer a similar service. They won't always buy as well as sell, that depends on the book you have and also the season. During the low season it can be impossible to get rid of books you have finished, because they don't want lots of excess stock on their hands.
One piece of advice I would suggest heeding is that when you do try and sell approach one of the Western owners because I have found they tend to offer you a much fairer price.
This shop is on the main drag of Ratchawithi Road just opposite the UN Irish Pub and so is rather convenient. It has a wide selection of books from the 60Baht well thumbed trashy novels through to brand new books. They cover everything from fiction through to guide books, spirituality, biography and maps.