The reason is that you have done the work to get educated, and many ebay buyers do not. They just assume that ebay is the best source for all things. There is probably some "auction fever" at work, too, assuming you are looking at completed auction prices....Not sure why given that with FFordes you get a little warranty too?
As for the 150, if I had to restrict myself to only one lens of my kit and sell the rest, my 150 would be the keeper. Buy it, you won't be sorry. As mentioned, though, you probably will want a short extension tube if you're looking to do tight portraits.
I would buy the 8 in preference to the 16, but I think the 8 is the most expensive one.