Iggy,
Based on all our replies here the consensus is to TAKE THE MONEY!!!Exclamation pointssssssss, end of story.
Most people here seem to be VERY much in agreement that the idea has already run its course and this is NOT a good indication that you might get anywhere near that amount on NP.
The longer you wait to accept the offer, the more time the buyer has to think about it and:
a) Find another domain name he will be equally happy with that might just be available for reg fee or at least less than 3 grand.
or
b) Discover this forum and realize his mistake.
Yes, it is conceivable that the buyer may actually end up making big bucks with the domain but it will be more because he works his butt off promoting it in some way and not specifically because of the "catchy" domain name (again, see item "a" above).