We just bought a new Oasis this past year (bought a 2011 in 2012, new, still in the box)
My one fear is that selling this boat is going to be tough. You need to find the right buyer who understands the value of the Mirage Drive. It's not a cheap kayak to buy new, and it doesn't have to be sold cheap used, but you'll need to find the right buyer.
One thing to promote in the ad, might be to say something about how you can kayak together but don't need to be in sync as you would with a tandem paddle kayak.
Two people of varying degrees of fitness will have no problems in a Tandem Mirage Drive boat. That can't be said about a regular tandem, and if two people of varying fitness buy two boats, they'll never be happy. One person will always have to slow down, the other will always have to try to keep up.
Sorry that I can't help you on actual values. If you can be patient, and advertise in the right spots, you'll get the value.