I'm guessing it was an adult kid. You don't know how many times I've heard of stories where parents decided to clean out their adult kids' rooms after they moved away and get rid of all their old toys, only for the adult kid to come back home and wonder where all their stuff went because they never wanted to part with them, including old toys their parents probably didn't realize had become collectibles.
I've come across a few deals where the item was too good of a deal (the person didn't know what it was) and my conscience wouldn't let me buy it without telling them they were sitting on a gold mine. IN this case I didn't realize this was the famous 3666 (I thought I had one, but apparently it's a different castle than I thought all along! now I have to find out what castle I actually have!), but if the person doesn't really care about it, I couldn't resist the $40 deal (I looked it up on ebay though, and saw that depending on completeness it recently sold between $53 to around $90). So $40 didn't seem to far off to me (also considering they told me it may be missing some walls, possibly) and had no accessories or clickies included).
Sadly though, I now want this castle some other way. and i Must find out what castle I actually had all this time (the steck one that I cannibalized to add to my knights kingdom castle). Sorry, I'm a castle noob.