Well, if I
personally had an autographed version of The Satanic Bible I
would probably never sell it - except maybe for a extremely good price.
Everything (like everyone) has it's (their) price, and I personally can't see why it would be a problem to anyone except the buyer.
You noted that you have nothing against collecting such items: If no one would be willing to sell their own items, collecting them would prove to be a tad problematic, wouldn't it?
In a nutshell: I might not sell such an item, but it is not my problem if someone else wants to. Hey, at least they're making a buck and some Satanist (perhaps) can get an item he cherishes. Win-Win, I'd say.