There are specialist sites for recovered 2nd hand laptop spares, tend to specialise is specific makes, but google normally finds them. Or Egay.
Yeah - I've bought Compaq spares like that in the past.. my stock of Dell bits came from old stuff work sold off that were non functional.. I bought 15 laptops and ended up with two working ones and a giant pile of bits 
System boards seem to be the buggers, and generally not cost effective - when a system board costs me £100, plus an hour fitting (inc testing), making it a minimum of £135, its not cost effective for people, buy a new one instead.
Yeah - everything moves on so fast now.. still, back when I was self employed I don't think I ever sold anyone a system - just fixed whatever they'd buggered up in Windows, usually 
I got into it early, maybe around '90/91, so got business from people buying their first PC, and the word of mouth (as PCs were considered a black art then), then all the upgrades for Win3 and 3.1, then same again for W95, then all the upgrades when the internet start yo get popular around '96/97.
After that, the arse dropped out of the market, and the likes of Dell made it not worthwhile.
By the millenium, everybody thought they were an expert, and were not prepared to pay my rates, so when I moved in 2001, I shoved all my customers onto a mate, and never really bothered. I just mostly do family and friends now.