Do you have the instructions and the packaging insert? I think all of these were sold in sealed plastic, so since you said they are assembled I know you had to destroy the packaging. If you look at the price guide on Bricklink, the Falcon sold for, on average, $20 used, $38 new (I'm guessing that means in the sealed container). So I might try to sell them if I were you. I agree with Sschmidt above that these particular sets are not great for castle parts - mostly gray wedge plates IIRC. Ironically, the one that I appreciated as a castle guy was the (IMO) by far worst model - the MTT is an old brown slope parts pack. If you did sell the whole group together on e-bay you might do better. There are some crazy 'buy it now' prices, but I doubt those will get takers, but you still might get some good bids from SW collectors.
BTW, if you're looking to trade these for castle sets, you might consider going on FBTB or Eurobricks. You might find more people there who really want to fill in their SW collections, and are also more willing to part with their castle sets.
Also, you might consider either BrickTrader
. I've not used either of these sites, though, so others would have to weigh in on their merits. Hmm, I'll start a thread over in B/S/T on the various sites.