Not worth $600. And even if you got him down to $600, you'd have to spend another $60 or so to replace the PSU. I'm also naturally suspicious of people who make grandiose claims for what they're selling - you're not playing Witcher 3 or Fallout 4 at 1080p Ultra at 60 fps with a 750ti, so even if it's not an outright lie (he can always say he meant 720p), it's misleading given that 1080p is the mainstream standard right now.