The problem occurs when you need a certain part, search for months and eventually make do with something that's not quite right. Then the part on question, the one you need, turns up for the bargain price of $65 so you buy it. Then the same part, that you no longer need, turns up for $25 and you can't resist a bargain. Then it turns up for $12...

If you buy all of them you can average your cost.

Exactly! With every subsequent purchase, average unit price goes down. But it's throwing good money after bad.

I am just beginning this phase... buying things like seatposts because I'm SURE i will get a frame that will reguire it !

Not only do I. Yu stuff in case I need it, I do it more than once, forgetting I already have it. D'oh! I'm trying to stop and am having a little success with it. And I've finally started to take inventory so I can stop buying and start selling. I've also started to tell myself that I can buy things when I need them. Not much point in stocking up ahead of time unless there's a very special reason.

i buy spare parts for my spare parts.....

Depends on the components...

I like to have an extra set of aero levers on hand.

Just cuz.

It's come in handy more than once when I've changed direction in mid-build.

I've been building up my parts stash of C&V parts for my C&V bikes since no LBSs in the area save anything 'that old'. Go into any shop around here and ask if they have any freewheels and they look at you like you're speaking Martian.