It should be perfectly legal to sell DRM-encumbered books, CDs, what have you. However, it should be just as perfectly legal to circumvent these restrictions. There might be exceptions, antitrust is one possibility, copyright misuse another, but in general, I have nothing against DRM-encumbered items. I oppose legally enforcing that DRM.
My last thought on the topic, for now: If DRM is applied to an information work, I agree with Ernie that circumvention should be OK. But how can (and should) courts identify the “thing” that is encumbered by DRM? How do we know that it’s thing-plus-DRM (where circumvention should be OK) and not just limited-function-thing (where the producer would argue that this is its “natural” state, and there is nothing to circumvent, and using the thing in some unintended way should be illegal)? We could say: If we buy it, we get to use it, circumvention required or not, and I’m sympathetic to that, but where does *that* authority come from? That’s what I’m struggling with.