I do not think that these that are being sold in ebay are all original. If playmobil had lots of parts in DS then maybe sellers used to buy from DS, assemble a whole new 3666 and resale it. Of course there is always the possibility of building a 3666 from various other sets, or what was left from them, and even add to that from DS....
I've bought several 3666 (ands 3667) castles off eBay.co.uk, and have no reason to believe any of them were assembled from parts off DS. I wouldn't care if they were anyway, as long as I get what's in the auction picture(s).
It would cost more than the original castle to buy all the separate parts, not all the parts are available on DS, and it just isn't worth someone's time and effort when the eBay sale price can vary so much. On eBay US used set recently sold for $152.50 (starting price $39.99 18 bids between 8 bidders), while a complete one MIB didn't sell for the asking price of $99. Another used set with some extra figures and horses was asking $150 for an opening bid and didn't sell.
If the original box is included then it's pretty obvious it's an original set. All the sets I bought second hand have signs of play, dust, smokey smell (eeew) or damage, often figures and accessories missing or replaced with different figures. Very frequently extra bits are included as the seller (either the parent who originally paid for the Playmobil or the child who has now grown out of it) has no interest in sorting the pieces they have into specific sets. Castles assembled for the auction picture sell better than a box of pieces, but some sellers just sell individual pieces from more than DS charges, so that'd be the way to go if you're trying to make a profit.
Much as we all love the 3666 castle, I don't think there's an underground industry in reassembling these castles from new parts. Bear in mind this set was first released in 1993, so any kid who was 10 then is 27 now, and most people lose their ability to appreciate toys and eventually dump them, if possible for profit. The number of 3666 castles appearing on eBay will eventually slow to a trickle, but for now there are still enough going up that a single one isn't going to command a massive price.
