From what I've heard, they usually throw out books with labeling.
I have found BC books in secondhand shops, but it does depend on the shop. I walk along the shelves scanning for the BC spine label. One shop I went into I mentioned that I was looking for BC labels, and then explained BC. The owner pulled a labelled BC book out from under the counter. He was about to take off the labels, and he said he always did. After I explained BC to him, he said he wouldn't from now on. Sceptic as I am I didn't believe him, but I smiled. He was telling the truth though, or although I didn't buy the book - it was in German - I did take the BC number and log it as seen and say where the book was. Some months later the book was logged in another city, so the book shop owner was telling the truth. He didn't remove the labels...at least for this book.