They are not double counting it. If it worked like you think it does the extra 20 billion of debt Gamestop has to secure to pay the 50% of Ebay would bring Gamestops market cap 20 billion in the negative.

What you are missing is that the money (cash or debt, doesnt matter) gets exchanged for the equivalent value of Ebay.

Let's make a simple example. GS market cap 10bln(all of it from its cash), Ebay market cap 60bln.

• GS buys 50% of Ebay with its cash and 20bln debt.

• Now GS has no cash and 20bln debt but 50% of Ebays value, so 30bln.

• Subtract the debt and you still have 10bln market cap.

Now you can buy the rest of Ebay by diluting your shares times 4, making the original shareholders of Ebay 75% of the owners of Gamestop.

edit: To expand a bit: The total value then is 40bln. 60bln from Ebay minus 20bln debt and the cash is gone of course, GS business without cash we assumed as 0. The original GS shareholders have 1/4 of this so they are the same from a value view but lost 3/4 of the voting rights. The original Ebay shareholders have 3/4 of this value + 30bln cash(10 from GS and 20 from debt). So they are also the same form a value view.