Is it? You will never know untill you try to sell it.
Re-read his link. It was owned by author Douglas Adams. Of course, he doesn't have any ultra-rare files on it, and I'm sure he has reformatted it so that it no longer has any trace of Adams-ness on it any more, but he does have confirmation from Adams' widow that it was, indeed, their computer. (Although part of the reason for so little Adams-work was that the computer was primarily used by Adams' wife, a lawyer.)
If it had actually been Adams' primary writing computer, and contained his drafts, it would be easily worth a couple thousand dollars. (Of course, then you run into the copyright issues.) But without Adams' direct work on it, it may be "priceless" to a super-hardcore Adams fan, but it's worth the same as every other IIfx to everyone else. (I'm a huge Adams' fan, complete with autographed, leather-bound, gold-foil-edge copy of the Ultimate Hitchhiker's "Trilogy", but I still wouldn't pay very much for it.)