On this saw, I think 16" is too short of a bar for a 64cc saw. I'd want a 20" on it. If you only cut stuff less than 16" then probably a 50cc saw would be a better option.
That said, this would be something to put on the shelf and look at from time to time. This series is popular and there will be people that want to buy one because they don't like or trust the 391. So I'd be very surprised if it doesn't sell for what he's asking for it. If he came off that price much at all, I'd be getting it if it's something that you want to store or use. If you're not into shelf saws, and don't really have a use for it, then leave it be as there probably wouldn't be much margin in it.
The saw was about $500 new, the chains I'd add $10 each for because they're probably safety chains and the case would be worth about $30 (yeah, some of this stuff costs more)
So if you could get the saw for $50 less than he's asking, you would have a no risk investment because you could always get that back out of it. I sell used ones for $350 with a 16" bar...
Note: this is a later MS390 since it has the decomp valve. Some of the early ones didn't have decomp.