It is a lack of supply and an increase in demand. If it was just flippers then they wouldn't be selling so fast or easily. I don't disagree that $400+ is a very high price, but the market seems to be bearing it. Whether or not it will last is anybody's guess.
Personally I think it has less to do with "uneducated" buyers and is more so a sign that people are simply willing to pay more. I think part of this is an increase in buying power as more people in my generation ascend financially. It certainly is in my case. I admit that I paid $400 for my copy, and I would do it again. (Allow me to now duck, to avoid flying objects being hurled at me.) I can afford it, so why not? I would have rather paid less, but at some point I just accepted it. Some of you probably think that is bad for the market, but I just wanted the game. I would rather have Beyond Shadowgate than $400. /shrug