That's the nature of rebuilding machines from parts. No one makes a profit building a Corvette from junkyard parts. It isn't going to happen with machine tools either. The value of the sum of the parts far exceeds the value of the entire automobile, machine, company, whatever.
But for those of us who are hobbyists this is still cheaper than buying a new machine of similar capability. A modern South Bend Heavy 10 (which is nothing at all like an SB heavy 10 built in Indiana years ago) is listed in the 2016 Grizzly catalog at $7500 plus $150 flat rate shipping. A 40-year old Craftsman Commercial 12x36 with a pile of accessories and tooling can be had for less than half that. And Grizzly's H-Mill isn't half the machine these Atlas mills are.
The Ebay seller's prices are high and continue to escalate because there is always someone willing to pay more. A few short weeks ago I was looking at a MFC Mill overarm bar support, M1-61 and I balked at paying $199.99. It sold on Feb 20 for the asking price. Two days ago another sold for a Buy Now price of $275, a whopping 37% higher in only a month! It is just a matter of market dynamics. Many of these parts are rare and there are more prospective buyers than there are parts, and those buyers are willing to pay the asking price.
I have noticed many Atlas/Craftsman lathe parts that were sold at auction on Ebay a couple of years ago now command higher Buy Now prices and they sell! Sellers are not amenable to the wide fluctuations of auction prices so they are leaning more towards Buy Now prices and all they need do is just sit and wait.
Spiral_Chips