I generally like to distinguish between vendors parting out old and new kits:
It can be extremely frustrating to watch a mint rare kit, you've wanted badly for a long time, parted out. However it's "demise" can in fact benefit a number of builders/collectors, who's been searching for "that particular part" to complete their build, restoration, or incomplete/damaged kit.
For instance, I like to build early/mid-sixties' customs using vintage parts from the various AMT "annual" kits long out of production. Sure, you can come by some of the parts in various "lots", but it's usually hard to find a complete set of matching body extensions, plated parts and glass, unless they've been parted out as a group. My latest purchase was a complete set of custom bumpers, grilles, and lenses from AMT's '62 Mercury Monterey. I've been looking high and low for those exact parts, and I'm very thankful to finally have them to complete a build I've been planning for years.
However, I definitely wouldn't offer any discount on a vintage kit, if I knew the buyer was just going to part it out for a quick profit.
As for new kits I rarely build a model straigth from the box - To me, searching for the "right" parts is part of the fun!
In recent years I've had a number of ebay-vendors specializing in parting out new kits in large volumes bookmarked.
Though buying individual parts/parts groups may be relatively expensive compared to buying a complete kit, I prefer to buy my kitbashing-fodder in bulk.
That way I'm actually able to save considerable amounts on postage - with the added benefit of not having to deal with large numbers of incomplete parts kits of limited value and/or interest to other modelers.
Having said that, it is annoying when Ebay searches are cluttered up by multiple results from vendors (re-)listing individual parts from current kits of limited to no interest to anyone (judging by their consistent lack of actual sales).
Just my 2C