Do libraries traffic in used books by way of Amazon Marketplace and similar channels?

I don’t think so. My dad buys and sells books on amazon. He has about 5,000 currently online. He buys many of his books at library book sales. I don’t recall him ever sayng he has ever had a book purchased by a library. I think he would have mentioned it, because it has a little bit of irony.

But, I don’t know for sure we need a librarian.

Some ofd them do but not all. But I doubt that they sell them online. Too much trouble. One of the libraries in my city has two huge used book sales on site each year and they also have an area in the building where they sell used books all year round. And the post office has a Library Rate but other sorts of non-profits are eligible for that rate too.

Did someone ask if there was a librarian in the house?

Generally not, in my experience. Books are bought new, and deleted or unwanted donated books are given away – in our case to a program called “Books for a Better World” if they are in decent enough shape. if they are moldy and stinky, we discard them.

Some local libraries hereabouts sell donated paperback books for 25 cents. People return the books and then they are resold. Adds a little cash for the library’s coffers.

My local library sells books it’s withdrawn from circulation but it sells them on its own premises, not online.