I think you are right in Many points here.
Definitely.
Just let me point to some things, where you may be not:
I (for example) don't buy Buk-collectibles because of the money (or value) they might have, when I'd try to sell them sometime - because the few I dare to buy will Never be sold (by me). Not even in the worst of economical crises. One of the few things I've learend is, that a good sale may pay the groceries for some time - but after they're eaten up, you're left with nothing. [of course MM wasn't left with nothing after selling his collection and he obviously kept some of the most personal items anyway.]
and yes, I too think about the (future) 'value' of my book with that painting - and will feel very good, if someday someone would offer me 5000.- or even 10,000.- for it. Still I wouldn't sell. Tenthousand are made very fast sometimes - and equally as fast they are lost. But a Bukowski-painting you can only lose once. So, the thinking about its 'value' is more a theoretical one, a sort of satisfaction. But not necessarily an attempt to sell one day. [I know at least ONE case, where it isn't, you see.]
signed copies surely ARE adding to the value - but sometimes they don't do this for the market but only for the owner. That's why I always prefer an author to sign me a copy with some personal words dedicated to me personally. If I'd intend to sell them, I think a 'plain' signature would be of more worth. (except I'd be a John Webb or Frances Smith of course.)
... on saying: "His books are worthless unless they are signed." - I kinda can see him stating this, but I do definitely NOT see him believing it:
He was such a Hank-fan, even in his last months.
I really think he was obsessed.
In July (just 2 or 3 months before his death) he literally [!] said to me, while we were sitting in his kitchen: "I think Hank was impeccable. He Never talked shit. He was the Smartest and Best. He was unbelievable. He Always helped me." (mediocre translation by me.)
And I'd like to add - these were nearly the Only positive words he had to say during that weekend. [except - and I say this with a certain pride - after he was complaining (again) about all the assholes and suckers and stupid people in the world and I said, I could be one of those, he just said: "If you were, I'd have recognized by now."]