Zip Books is a grant-funded program from the state of California, and the standard disposition of those items is for the book to be added to the collection once the original requestor has returned it. This is what we call PDA; patron-driven-acquisition. A lot of the "anytime" ebooks you'll see on Overdrive have been purchased the same way; the library will have their policy set to convert to an unlimited license if a certain number of requests for the item hit the system within a certain period of time. (though Zip Books are paid for by the State of California, not the local library system.)
Library ebook purchasing terms generally do not allow them to be lent outside the client system in their entirety, though some allow chapters to be supplied as copies (which is often the case for non-fiction items, where a patron only wants a certain topical section.) Most library electronic book systems use Adobe Digital Rights Management to control lending terms, and most often use a one-patron-at-a-time-per-licensed-copy purchasing model, though metered-access PDA purchase models are available. A notable exception is HooplaDigital, which is fully PDA; patrons of client library systems have access to the entire catalog of the vendor, and the library pays via transaction fees. Most library systems do limit how many Hoopla items each patron may download per month. (Hoopla contains a lot of indie books; they pay royalties directly to the author off each transaction fee that is collected.)
Here's the thing about returning digital titles that's a problem for me: publishers don't refund the money. They don't guarantee content. Books have always been and still are purely caveat emptor; there is no refund from the publisher if you don't like it or if you misunderstood the content. (Same for movies, too; you can walk out, but the box office won't refund your money unless everyone had to leave.) There is no law that says that a publisher has any obligation to offer a synopsis of any title, let alone an accurate one; book jacket blurbs are there purely for marketing purposes, and will always be chosen from the best bits. Personally, I'd love to get money back for every book I've purchased that my patrons didn't read, but it doesn't work that way. Being an indie author is a total gamble: marketing and WoM are all you have to make back your investment, and marketing is expensive, so you'd better make your product good enough if you want to break even. Reading indies is a gamble, too; if you don't want to deal with that risk, then don't take that leap and buy. Publishers only refund for functional issues, such as missing pages, corrrupted file structure, etc., and IMO, so should any bookseller that is acting as a publisher (which includes Amazon.) Having distributors paying refunds that publishers will not back warps the system, and will eventually cause it to collapse.