Seller not knowing combined with common approach of removing hard drives to either sell separately or because of data wiping. The CMOS battery error could have confused the final seller.
It's quite often that such finds come through multiple hands, and by the time it hits eBay/whatever the seller has no idea what they are selling except maybe for what's written on labels and can be decided in 5 minutes from looking at the wares :)
That's how I bought my (former now) AlphaStation 255/233, I noticed the case being sold as "old server case for use as HTPC" :)