Your best bet would be to find someone local if you can(craigslist maybe?), or to hold on to it until you can sell it in person at a ponymeet or the pony fair. The shipping on that sucker is going to make it *very* hard to unload. You could also try to unload it at a consignment shop if you have any in the area, put it out at a garage sale, or look for one of those "mom sales" that pop up every now and then. Basically it's kind of temporary consignment sale for people with kids. They bring in the stuff that their kids have outgrown, price it how they like, and then people shop the sale. They're rare, but they're usually only one day to one week long, and they have a large concentration of parents looking for a bargain, so an incomplete playset would be likely to sell.
If you just want it out of the house and you're not set on getting the most for it, I'd sell the accessories here or on ebay, and donate the shell to Goodwill or something. There are some people who would like the smaller items from the castle, but don't want the whole castle. Shipping on that castle is going to make it a VERY hard sell online. Not having the pony/ponies included is going to make it even harder. People buy playsets for the ponies all the time, but something that size that's missing the big collector draw is too hard to store and too expensive to ship for any collector to want it.
No matter how you try to sell it, you're going to have to price it to sell if you want it out of the house. It's just too big to ship, there's too many MIB ones available right now(and for cheap), and you took out the best part to keep for yourself:/ I'd say if you can get $20 for it, you're doing *incredibly* well. Even at that price point, I wouldn't expect people to be beating down your door to get it. $10 might tempt somebody.
I had the same problem with several Butterfly Island playsets. I am a Costco member, and when Anchor's Away came out, I bought playsets for several people who wanted the pony. They didn't want the playset though(shipping and space issues). They paid my cost for the playset, I sent them the pony, and NOBODY wanted the playset. I ended up with a stack of the stupid things. They ended up being donated to Goodwill when I moved. I just couldn't unload them:/ Shipping man. Nobody wants to pay that much.