Here is a new one for the 2015 year. http://www.austinkayak.com/search/Hobie%20Oasis%20Tandum
, go to this web site and see what a new one would cost.
The main thing before buying, would be to make sure there are not leaks. The only way to do that would be to get to the water and play with it for about an hours or so.. Then check inside to see if there is any water. The other expensive part is the mirage drives. There should be two and make sure the condition of the drives are smooth and squeak free? No grinding with the movement of the mirage.
The model will most likely be out of warenty. but at that price it is still a good deal. my opinion... Good luck