As to why Doc (and others from whom I've heard the same thing) doesn't sell any books, I would suggest it comes down to two very simple (but not easy) things: trust and convenience. Trust in this context does not just mean you believe someone won't blatantly rip you off (not very likely), but that you trust the process of buying from them won't be a hassle, that, if there is a problem, they'll try to fix it, that your book will probably arrive quickly, and that the price will be in the range of the lowest available on the web. If you've bought stuff from Amazon a couple dozen times before, as a lot of us have by now, you don't have to question any of these things, which isn't true across the web (or from vendors in the real world, for that matter). So the impulse to buy has few barriers. Secondly, the simple task of having to enter your billing and shipping info (yet again!) is a big thing that can stop a sale (at least for me). And that's to say nothing of the power of 1-Click. In a way, I guess these two things are really about positioning and UI -- but in a different way than stated before.
Oops. Dave gently reminded me that UserLand was doing RSS syndication and aggregation before any of the companies I mentioned below (so I added a link to him). I told him it wasn't a conscious oversight, it's just, unlike those other companies/services, that's not how UserLand is positioned in my mind. Ironically, Dave knows a lot about positioning. In this post, he argues that Doc Searls doesn't sell any books because of positioning, not, as Jakob Nielsen claims, because of user interface. I think that's a big part of it. In fact, I wrote a piece (which Dave pointed to on Thanksgiving Day 1998 -- unfortunately, the column is no longer up) about how Amazon was ruining its brand by getting into all these other products. (The jury's still out on that.)