It's worth more than $400 imho Jason, but how much more is the $400 question. If you have someone "hot to trot", then some subtle
but sensible counter offering should see you get a good quick result. Particularly if you are not greedy.
Definitely not worth $6800 (at least not to any rational domain buyer).
Which brings up a good point, who is the buyer? what do they do? and what do they want to do with the domain?
The above is always good to know, so you can work out how deep their pockets are and if they have room to negotiate or if they are running a thin margin business.
If $400 is their initial offer, I think that's a great first up offer, shows they are pretty serious. I'd counter with something in the high $XXX maybe even $X,XXX just to see if they are the sort of person is willing to negotiate.
What's the domain worth - from my perspective it's a low to mid $XXX - but that's from my perspective and obviously it's the buyer who is willing to part with their cash at the end of the day who determines value.
However given that it isn't what I'd call a "keyword rich" domain I think it'll be A LOT of effort to find another buyer who'd be willing to go $500+ on it. So that's probably worth considering also.
I say make hay while the sun shines.