Full disclosure: I'm a nerd. I like technology and also trying to fix things. Sometimes against my better judgement. I've built 3* computers and tinkered with many.
I've been keeping my eyes open for people giving away broken computers, selling broken ones, or selling working ones that are very inexpensive. I have some minimum specs I look for, but overall if I can get my $$ back in parts then it's usually a decent prospect. I have a few sitting around that have become parts sources because they're not worth trying to fix or too outdated.
A few weeks ago, I bought 3 broken laptops for $40. The person had them sitting around, covered in bumper stickers and other stickers. One was in rough shape, but the other two looked fine. 2/3 of them had chargers.
I got them home, plugged them into my universal laptop charger ($25 investment), and tried to boot linux from a thumb drive. ALL 3 of them booted right into it!
I had to do a bit of legwork though... I spent an hour with Goo Gone and an old gift card scraping stickers off that wouldn't just peel. One was missing a hard drive and caddy, so I pulled a drive from another machine and got a caddy/cover on eBay for $5. Replacement touchpad sticker (the texture of the touchpad on this machine) $3...
I used the original Windows licenses that were on the computers, reinstalled fresh copies of windows on each, and let windows update run up to current.
Total time invested: 2-3 hours if that, if you include grabbing them on my lunch break.
Total money invested: $40 for the laptops, $8 in parts, and I'm allocating $30 of cost from my parts laptops to this endeavor for taking one of the power cords and one hard drive from my lot.
Based on trade in values at a local retailer, I could get ~$220-$230 for the 3 laptops. I'm going to sell them myself, and expect $140 for the one that needed parts, and $80 for each of the others. I'll net ~$220 on the conservative side of profit.
Note: It is not always like this. The parts laptops I have were $20-$30 each, and if not for the chargers and hard drives, are essentially worthless. Sometimes you get a dud where the specs say it should be decent, but in reality it takes 3 minutes to boot into a barely usable linux. Others may have decent specs but a cracked screen and lack a charger. This is always at your own risk.