I would never purchase a Disney Reward card. As stated above, if you were to close out the account they would no longer be worth anything, and I don't really think it would be a *permitted* by the card terms activity. I think you may have better luck getting gift cards and trying to sell them, if you are going to sell anything.
That is not to say you cannot sell the rewards, just saying I am not sure there is a going rate. I have never heard of anyone doing it before. It is why we have switched to a cash back credit card though. At least cash can be used on whatever you want, whenever you want. Not just when you are going on vacation.