The only input I have is that eBay is weird.
I've got a few interest items that I track on a regular basis. I have, for instance, a fairly complete spreadsheet of every Altair and IMSAI that's sold for the last 18 months or so. Of those items, I see wildly varying sale prices that can't be easily explained by the condition of the item or the description for the auction.
For instance, I purchased a Sharp PC1500 setup including the printer, case, all docs and a small pile of other goodies for about $25+ shipping. I have since seen just the unit itself sell near $100 and other setups with the printer sell for $60+
For Altairs I've seen prices range from the sub $1K range to $3K+ with some of the lower priced machines comparing favorably to the most expensive.
So, if you're interested in getting top dollar for the machine, put it up on eBay with a safe reserve and see what happens. There's a sucker born every minute and maybe you'll latch onto one!
Good luck!
Erik