Buy or Sell?
Are you looking to buy or sell? The answer differs based on that.
As you learned from eBay, you are free to ask however much you want, but that does not mean you will get it, which is why checking prices on eBay is only really helpful if you are planning on undercutting them all.
The asking prices do give a buyer a clue as to how much one can expect to pay. It's really a matter of how badly you want one and how much you are willing to pay. I can say that it seems that one will have to pay at least $350 and ought to be able to keep it under $500. Hard to say much more precisely. I am not aware of any site where you can figure average purchase prices, you can only see asking prices.
I also think that I would only be willing to buy one that is still in an unopened shrink-wrapped box. Fortunately, I bought two of them at Best Buy the summer before they were discontinued in anticipation of that happening.
Quite frankly, with the existence of a 128 GB iPod Touch, I would certainly not pay more that $400 for a Classic. I would make due with 128 instead of 160 and be glad to have no mechanical parts to worry about.