The only prices that matter on ebay are the final hammer price on completed auctions. There is a huge amount of vintage computer gear listed at fantasy prices that just gets listed over and over with no bids. You can't look at those to determine the value of an item any more than you can go by the starting bid. When I sell stuff on ebay, I almost always start it at 99 cents and let the interested bidders duke it out. Most items seem to receive a few low bids early on, but the real action doesn't take place until the final hours. Final prices are all over the place too, you might see a really nice item go for $50, then a week later an identical item in much worse condition goes for $250 or vice versa.

For what it's worth, the cheapest 68K Macs tend to be the stuff that was prolific in schools and the lower end consumer stuff like the Performas and such.