When is a library no longer a library?

Here is an existential question - What happens when a library sells books?  Are they still a library or are they a bookseller?   This blog posting advocates having libraries sell ebooks directly to those who don't want to wait to read the latest bestseller.  One commenter noted that, "Then you're no longer a library if you start selling books in volume. Libraries loan books."  That begs the question - when is a library no longer a library?

Certainly, libraries have sold books before, usually used books pulled during weeding projects or donated by patrons and others who were cleaning out their own shelves.  This is legally possible due to the print materials' "first sale doctrine", which does not apply to electronic resources.  Do these sales make the library a bookseller?  Does it no longer make them a library?

What about selling books by a visiting author?  I'm not sure if this still occurs in public libraries, but I recall this many times in our school libraries.  If so, how does that affect the library's role in the community?

I was at first repulsed by the idea of mixing commerce with the library's ideal of providing resources free of charge.  But thinking about it further, I'm not sure it really changes this ideal - the library would continue to lend, but under the restrictions enforced by publishers.  Providing an opportunity to own a book would enable the library user to continue to see the library as a source of information and reading material.

What was it that has kept libraries on the edges of the bookselling business?  Has it been the traditional separation of commerce and government?  That separation is blurring, with private/public partnerships being advocated by the fiscally-conservative politicians and activists.  Has it been the reluctance of librarians to view themselves as commercial?  Business owners are often both revered and reviled in large and small communities.  As public servants, librarians (including myself) like to consider themselves above the level of those who sell.  But don't we need to sell?  OK, usually we sell the need for a free access to books, resources, and public space.  But how is selling ebooks any different from holding a book sale?

If the issue is the separation of commerce from government, then the Friends of the Library model should work as well for e-books as it would for used book sales.  Links in the catalog could direct users not to Amazon (directly or indirectly via Google Books), but instead to the FoL's ecommerce site where the ebook could be purchased.  FoL would continue to donate the money raised to the library.  Is this really inviolate?  Does this really change the identity of the library?  If the library still continues to loan material free of charge, isn't it still a library, even if it sells?  I'm not sure...