I just checked eBay to see what you're referring to. The sellers I see at the top of the page aren't just selling the basic set; they are selling the basic set plus expansion sets. (There are many different expansion sets, some of which cost as much as the basic set.) That may be why their prices are so much higher than the retail price for the basic set.
If you want a list of the different expansion sets, you can visit my (admittedly geeky and lame) website about boardgames:
Boardgame Freak . It might help you choose which ones to get and which ones to avoid.