Amazon and Microsoft have a reasonably polite relationship.
Where they compete in Business Services they behave like professionals and accord each other due professional courtesy. Since the overlap is minimal, there is no reason whatsoever for Microsoft to go to war Amazon, much less start such a war by wasting money buying the B&N storefronts just to liquidate them.
Microsoft only buys companies that do something they don't and do it well, or own a particularly valuable asset; B&N is neither. More, from a tech company point of view, there is zero net value in B&N; the plus of the Nook marketshare is negated by the minus of the storefront leases.
Even Nook alone isn't all that valuable to Microsoft since it is built on Adobe and Google tech. If MS had any inclination to get serious about consumer ebooks (which they clearly don't) they wouldn't be killing MSReader/LIT or they would be buying into Blio, which is built using MS tech.
As for Google buying B&N or just Nook: MS would *love* that.
Remember, among the other problems facing B&N, they are looking at a patent infringement suit from MS over Nook and they "oh-so-prudently" went publicly ballistic, petulantly ranting about MS's "unfairness" and effectively daring MS to do their worst. So anybody buying Nook inherits the ongoing lawsuit and the poisoned negotiation well.
Google, with its hands-off approach to Android patent lawsuits would have to think twice about buying the lawsuit, as MS has been careful not to sue them, yet. If they did, it would be the opening MS could use to amend the lawsuit to include all of android. And, unlike B&N, who only face monetary damages in a MS win, Google would be facing a cease-and-desist order that could force them to re-write Android, as well as the deep pockets to actually pay full freight on *all* the patents MS can assert.
Whether MS could actually win such a trial is unclear, but the legal costs, the further FUD on Android, and the stakes at risk means Google is not going to touch Nook until B&N settles the lawsuit. Remember, Google already faces a billion-dollar liability looming over Android from Oracle's case. They don't need MS piling in. It might tempt Apple...
Indeed, odds are, any potential Nook buyer will insist B&N settle the suit first, just to see how it affects Nook profitability.
Realistically, the most viable buyer for B&N (IP-wise) today would be Apple (they have wide patent crosslicesing with MS and no use for Android) but they have need for neither the storefronts or Nook. The college bookstore and textbook business they might have an interest in, though.
Other candidates? Sony would be a nice fit for the ebookstore, as it would get them out of the generic Adept ebook business and back in the walled garden business they started with, but they are bleeding money from a million wounds already; they can't afford the upfront costs. If B&N ever got to liquidation (unlikely as it is) they might be a good buyer for the ebookstore piece.
Hmm, basically, It looks like B&N as currently constituted can't be sold, but in pieces...?
Gulp! Maybe liquidation isn't unthinkable.
