If it is an old magazine, you can go to CR Vanguard Supermarket or RT Mart Supermarket. Larger ones can go to Yantan Book Wholesale Market.
If it's an old literary book, you can go to Wu Fengqi Bookstore (basement 1), which is at the northeast corner of Panxuan Road.
If it is an old study textbook, you can go to Chaohua Xishi Bookstore opposite the Yuzhong Campus of Lanzhou University and the Business School (I don't know if it is still there).
If it's an old collection, go to the Imperial Temple on Zhangye Road.
If it is an unclassified old book, look for a garbage collector. They have many there, hehe.