Them offering you 20k is just to see how you will react. NEVER accept the first offer. Could be very well that a major company is behind this offer, and the person working on their behalf is looking to fatten his or her pocket.
Point is, Its not your average joe shmoe who will buy that domain.
Try to evaluate how much revenue is done in that particular market and you will see what im talking about.
20k for a market that does well over 8 figures a year and is increasing is nothing.
I would definetly tell them to
www.gofuckyourself.com ;)