Answer:
"NO CHANGE"
Step-by-step explanation:
In the given sentence, 'but' has been used correctly. The use of 'But' shows transition of the thought and action. 'But' is the conjunction that is used to show contrast between the first action and the second action. The first idea is to 'no buy anymore books' and the second idea is 'stopping by the library's used book sale'. Here, the two thoughts are standing in contrast to each other. Therefore the use of 'but' to build a transition has been aptly used.