Publishers' website and other ebook vendors, like Kindle, sell single-user ebooks.
These are often loaded directly into your account or, for many publishers, accessible using a code that you receive after purchase.
Even for vendors that offer access codes that will be good for more than one user, these formats are still not linkable or able to be uploaded directly into the University Library collection.
We work specifically with library vendors who can sell multi-user and unlimited licenses that can then be linked directly into our library collection.