bmwm3186 :
That is the minimum requirements and you are not going to be able to run it on medium settings, How much do they want for it? if you want to play dayz build a $600 dollar computer. I will be happy to give you some recommendations on parts too if you want.
Technically speaking, the 8800GTX is above the minimum requirement of an 8800GT (though they do not specify what version of the GT, but they did not say the GTX). Because of the low resolution of the monitor, it would help the GPU to breathe easy (barely). You are pretty much right though, since its riding the edge. It would run a little better because of the monitor though, which is why I guessed medium. Here is a video of it being played on an 8800GT.
http://www.youtube.com/watch?v=vU2y3cQlW2s
I have no idea what any of the description of the game says since its in...Russian (best guess). No mention of the resolution or the settings I can see. Might be lower res since the video was captured at 360p.
I used to run old DayZ (Arma) on an 8800 Ultra and a AMD 965 Black Edition on decent settings. Just need to turn them down. I've also run the old one on a HD5770 and was surprised that it was actually able to handle it. I believe the DayZ standalone is coded better than the old Arma based one.
I'd be careful with computers from Craigslist. Sometimes they might be on the verge of breaking. That computer might not be too bad though. There might be better ones out there for whatever you are planning to pay for it.
As above, a new computer would be nice.