I'll take it apart piece by piece, if you'd like:
CPU - FX-6100 goes for about $70 on eBay, but I don't think they move very quickly because demand is really low. It's really a pretty slow CPU by today's standards.
Cooler - $25
Motherboard - Can probably assume $50, since that's the going rate for an entry-level AM3 motherboard when it's new, but this one is used
RAM - Equivalent RAM goes for $70 on Newegg, new. This is used.
Hard drive - You'd be out of your mind to pay $140 for a 2TB drive today, when Newegg sells them new for $50.
Power Supply - available on Newegg for $35 after rebate, new.
DVD Burner - probably wouldn't need this in a new PC anyway, but let's call it $15.
Case - $30
Subtotal before Windows, if you were to buy these parts "new", is $345. You can probably subtract around 25% since they're used, bringing the value of the PC down to about $260.
The fact that the owner is not going to wipe Windows off of it does add some value - a new license is about $100
So, you might be getting a $360 PC (including Windows) for $270, but I would absolutely not advise anyone to buy an FX-61xx today. You'll want to replace that immediately, and that's going to add to your bill. There are tons of people on this forum who are, at present, asking about upgrades for their FX CPUs because these chips aren't delivering desired framerates in today's games, and to do so would require a new motherboard. Take into account that your Windows license is not worth anything if you change motherboards - you can't reactivate it, and that added value disappears.
You could conceivably just drop in an FX-83xx CPU for ~$150 and call it a day, but at that point you'd still have been better off buying Intel parts and a new Windows license for the same money.
My 2 cents.