The open secret of eBook / eAudio sales is the buyer does not own the item. Rather, each item is licensed for use because digital and print are legally different. There are several licensing models that publishers offer when selling to libraries:
Not all titles are available in all models. Generally, a title is either Perpetual Use OR Metered Access. Publishers will selectively make titles available in the Cost Per Circulation and Simultaneous Use models. The library can choose the best option as offered by the publishers.