Mountain Biking - Depreciation on Mountain Bikes...

Ok, so I have actually have two questions.

First, after nearly five years of riding the hell out of my trek 4500 I've decided that this fall I need to see about getting a new bike. Mainly I'm just sick of getting the hell beat out of my knees on descents so I'm looking for a full suspension bike. I've been looking at the MSRP on the manufacturer sites and the prices listed on craigslist and ebay and it seems that three or four years out the price drops by about a third to a half. My question is why. Is it because the technology is vastly outdated, because they've been used hard, or simply because they're older?

Second, I've been looking at some frames on craigslist and the older frames sometimes have headsets, bb, etc left on them. Is this a good way to save some cash or would you be better off buying new (with a warranty and such)? I'm a poor graduate student but I'm competent enough to build a bike up...I think <gulp>

1. It's because they're used, and everyone prefers a virgin.
2. Headsets and BBs are among the tougher things to install and remove from a frame. Inspect closely if buying used. But, more importantly, check that frame for any slack in the pivoting mechanisms, as well as for crash damage. Also check compatability of the BB with the cranks you want to swap over.

I'm curious about the notion of using a FS bike to protect one's knees.
Do you mean because you crash more frequently on a hardtail?

Naw. I've got bad knees. It hurts when I run too. Even staying loose the impact is being telegraphed through the bike to them. Maybe it's just bad form but I thought I'd give the full suspension a try. Given your advice I think I'll buy locally.