eBay is worth $37 billion today. They very clearly were not being valued at a negative market cap.

There are no examples in recent US stock market history of a large, extremely profitable corporation with a positive balance sheet being valued at a negative market cap. There's a reason for that.

The combined unit was suffering a discount. That's precisely why PayPal was separated. If your premise were right, eBay today would be worthless, not trading for $37 billion.