Some people here that are enthusiasts, or buy bulk at auction, might see that as too much, but I think that's fairly reasonable. (Newegg wants over $100 for the CPU alone!)
So maybe I am overcharging, I don't know.
You're making a false assumption there. Just because the CPU is listed on Newegg for $100, doesn't mean it's selling for $100. I see it all the time with servers. Old (but still newer than the systems we are talking about in this thread) system listed on eBay for 10x the actual going rate. Just because you can find somebody listing it for $10,000 doesn't mean it's worth $10,000.
As I've said before, I understand the desire here. I used to do the same thing with servers. But eventually I saw the light. I know we butt heads and I'm really not trying to be mean here but you and walt are fooling yourselves if you think this equipment you are buying (and selling) are good deals.
Personally as a professional, I'd never sell a system that old to somebody. You're trying to milk a few more years out of a very dated platform by putting some upgrades in it. It's like putting new tires and a coat of paint on an old car with 500,000 miles on it then trying to sell it for nearly new prices. Yeah, it's nice and clean but at the end of the day, it's got very little life left in it.
Take your above system that you said you might charge $400 for with Windows. On the surface, that sounds reasonable. However, 5 seconds of searching:
http://www.newegg.com/Product/Product.aspx?Item=N82E16883280708&leaderboard=1
That system is 5 generations newer and obliterates the CPU in your system. It uses less power while doing it as well. That system is only a 5% cost increase. Even without an SSD I'd recommend that in a heartbeat.
Most the SSD's I've seen you mention cost you around $30 if memory serves. So $50 more (including SSD) and you're still only at about a 12% price increase and that system is basically a rocket by comparison at that point plus has all the new shiny stuff like USB 3 and UEFI. Your system only needs a dGPU because it's so old. The Intel HD4400's on the quads are more than enough for standard desktop usage and light gaming. I've played CivV and Endless Legend on mobile versions more than once.
At the $300-ish mark:
http://www.newegg.com/Product/Produ...kLogic-_-Desktop+PC-_-Acer+America-_-83101193
Still twice as fast in single threaded operations and still over a 50% increase overall.
It would be different if you already had the system and were just trying to keep it going on a budget. But selling 6 year old computers for anything more than garage sale prices just rubs me the wrong way personally. You complain about that guy ripping off your mom by selling her a 7 year old computer while you are happily selling 6 year old computers. Would you have been fine with it if she only paid $400?

Don't get me wrong, I know you aren't trying to rip people off. I just think you are overvaluing those systems.