Needing to reduce collection

[Sellingsomewhisky]

I am representing a keen whisky enthusiast, who has recently decided to reduce his collection of predominantly Islay Whiskies.
I have a list of about 40 bottles including rare and discontinued whiskies, such as:

Benriach 1966 Signatory Cask Strength Collection; Bottle No. 60/175;
Bruichladdich 1964 Gordon & Macphails; Bottled 1995;
and Port Ellen 1982/27 Murray McDavid Mission; Bottle No. 823 of 1175.

If anyone is interested in seeing a full list please contact me via private email preferably, and I will send it out.

Thank you.

[Mickey]

Could you explain to me what makes a whiskey really expensive. I mean, there must be more than age. And what makes up the quality of some real excellent whiskey? Which are the metrics taken into account when comparing whiskeys and looking for an exceptional bottle?