my take on this is that the sellers don't know what they're selling, moreso than are trying to be deceptive. They probably don't realize how detail-oriented collecting playmobil is, and that avid collectors will interpret this as a misleading auction, when, in reality, the seller probably thinks a castle is a castle is a castle and is all the same.
Unless they were willing to assemble everything to see what they truly had, incliuding missing parts and extra parts, what they should have done was just take pics of the bins showing the sheer amount of playmobil goodness, and then in the auction just listed it as HUGE LOT OF BINS of PLAYMOBIL, and in the description noted that they don't know exactly what parts go to what, but that there are many many parts that may or may not form complete sets, and surely would be a great find for a serious collector who wouldn't mind the trouble of sorting the lot him/herself and just taking their chances by bidding on the pile of playmobil as-is.
I know, personally, if I had extra money, I would bid on such a lot even if I wasn't guaranteed any complete sets; I would know there is a lot that can be done with it, and that most likely I'd have to spend great amounts of time later trying to piece them together and complete the sets, or use them as extra parts for existing sets, and/or re-sell the extras back on ebay once I've sorted them appropriately.
I really don't think the sellers meant any malice or fraud. Probably just ignorant of proper selling technique, and ignorant of Playmobil in general. A knowlegeable seller (even a fraudulant one) would not make these mistakes, IMO.