Actually, we don't know how "working" it is... especially for a single motherboard system with no IO. Trust me, it can look like it runs, but until you can load Microsoft basic over a serial port it ain't working. Now this one on eBay is in great condition cosmetically I think because it's only a 256 byte system, so you can't do much with it so I'm sure after the novelty of getting it running it was never used or it never worked right. Tends to be the case with non-restored machines, they are cosmetically perfect because they never ran.
As for price, this is actually low, not insane. Breker sold one similar over a year ago for 15k. We haven't seen a complete verified working early Altair in good condition setup with original motherboard(s) that can run basic sell publicly for years. The ones I have seen are either "A" or "B" models or have had the motherboards upgraded to single piece or something like that. The MARCH unit that we use to demo basic at the shows is like that. It's a very very early Rev-0 Altair, but the motherboard has been upgraded to a single piece PT board and the power supply has been upgraded. Very common at the time, which is why original is so rare.
So the winner of the auction got a great deal, but may have to do some work to get it functional. I'd pull, but keep the 256 byte card, find two early 4k MITS Ram cards and a SIO serial card, hook it up to an ASR-33 and load basic to prove it works completely. If not I'd get it to work. Then you can prove it really works and not just lights up blink lights. You can deposit memory and read it on a partially working system, so I never use that as a gauge that it works.
Original motherboard setups are rare and fragile.
Cheers,
Corey