Because my wife and I are building knife collections (RMK's, AMK's, and A/F 'Fighting Knife' variant's), we had long assumed that the sum total as a collection would have more value than that of the individual parts...
Unfortunately, our experience now shows that collections, as a package deal, usually sell for much less than one that's parted out.
Generally speaking, collections are sold for one of just a few reasons: The original collector has passed away, and the family/executor doesn't know the value; the collector has lost interest in collecting; the collector has become financially insolvent and needs the money quickly; the collector has become sick or infirm; or the collector has gone through some traumatic situation such as a medical emergency/divorce.
As such, most collections sell for much less than they are actually worth. Finding someone with the exact same interest as yours, that has the money to pay full value, at the time you need the money, is a rare occurance indeed, especially in tough economic times.
Additionally, many collectors remember what they paid (within reason), and haven't tracked the increase in value, if any, over the years...this is actually very common, and explains how many purveyors are able to make enough money off their purchases and sales to stay in business.
We now know more than just a few advanced collectors that are slowly parting out their collections, at one knife a month or so, for extra income. This would appear to be where the 'smart money' is...YMMV
Hope that helps,
Mel