Personally, as someone who's obviously interested in true crime, I don't find such collections apalling if the items were obtained via legal means. I also don't think selling murderabilia should be banned, unless the proceeds go to the criminal, as they should not benefit from their crimes.
That said, I wouldn't own any murderabilia items myself, as I don't want to bring any negative energy into my home. I can also see how murderabilia could upset the victims' survivors, though.
Of course, depending on how this particular collector is portrayed tomorrow night, I may have more to say about him. On the previews, he did seem quite a bit more excited at being in the same room with the Unabomber's stuff than I would. Almost like he was meeting his favorite actor or musician...very fan- or groupie-like.