fights with strangers

I hate getting into fights with strangers. I really do. Road rage, random exchanges in stores that lead to words -- all unnecessary strife with people who you have no beef with in reality.
Craig's List is the virtual facilitator of some of this needless grief. I'm currently embroiled in an altercation with one Insiyah from whom I "purchased" an original nintendo as a wedding gift for a friend (um, yeah...I'm awesome).
I forced Ins to bring the Nintendo to my apartment since she wouldn't let me come to hers, because I wanted to be at a place where I could test the machine out before paying her for it. The machine didn't work, but I was leaving on a plane later that day for the wedding and wasn't sure if perhaps the two of us, in our basic technological ineptitude, couldn't figure out how to plug the thing in properly. Insiyah graciously allowed me to take the machine without taking money from me, and I assured her that if it did work after I tested it in L.A., I would pay her for it. Turns out the machine was missing a certain cable (pictured above), and my brother happened to have an extra one lying around (a fact which, in itself, we should pause to appreciate). With the cable, the machine worked perfectly.
I was willing to pay I. for the machine, but as it was missing a piece without which it wouldn't have worked, I didn't think full price was fair (which was 40 bucks, but that's not the point). She's standing firm on the 40 and that's kinda pissed me off, on principle, so I haven't been responding to her e-mails and haven't sent a check, and she's threatened to take "further action," which is kind of hilarious.
I intend to send the check, but I want to piss her off for being a sucky bitch, which apparently I've succeeded at doing since she's threatening to sue me or whatever. And the reason I've relayed this entire story is because it's an instance in which I'm not actually sure who's right and who's wrong (though I concede that I'm being a big baby in withholding the money). Still, it's not just senseless road rage at work here. There are principles when it comes to buying and selling, dammit! Ah, Amazon buyer protections...Craig's List could use 'em.