I think your comment illustrates the fault line between a "collector" and an "investor." Most of the old time "collectors" I have met are, when it comes to comics, hoarders. They aren't investors. They could cash out for big bucks, but most would rather take it to their graves. They can't even understand why anyone would want to sell the items that they put so much effort into finding and which cause them so much pleasure. Which is probably why some of the more prominent collections recently sold at Heritage have been estate sales (Roberts' original art sale, Weist, etc.).
I also think that's why you see some books coming on the market repeatedly. They haven't found a home with a true collector yet. They are just flitting from investor to investor.
Not to mention if you sell then A) you no longer have a book and likely won't replace it and B) if you sell now, you lose out on future gain in value.