It's not exactly for free though, since you have to do the work of breaking the bookshelves. It's like you have a manufacturing job, you buy something, do something of value, then sell for a profit. I don't think it's a bug.
I do not think that breaking bookshelves is worth getting emeralds. The fact that the infinite emeralds are only available from discounts, but not from the original trades is also evidence of it's likely hood of it being a bug.
I do not think that breaking bookshelves is worth getting emeralds. The fact that the infinite emeralds are only available from discounts, but not from the original trades is also evidence of it's likely hood of it being a bug.