I noticed the FW400 also, but a couple of things about the add have tweaked my interest.
They originally posted them as Power Macs and updated their add at my suggestion.
The buyer must pick up only from a downtown office location.
Finally the add mentions "dual core or better".
This all suggests they are not sure what it is they might have, so on the off-chance they maybe purchased 1 MP initially and then added others later (newer models), there might be hope here for a good score.
I'll keep on them for more info with this in mind.