My thinking is that there's no way he'll part with it for less than around $300 (he's clearly not a n00b), but if you have to pay much more than that then you may as well hold out for a nicer frame.

By the way, I recommend checking out the TT length first. A lot of NJS bikes are a lot longer than they are tall, and from the pic it looks like that might be the case with this one too. So that 56cm sizing might end up being pretty deceptive.

Anyway, if it fits, I say give it a shot--looks like a really fun project!