As mentioned below, enterprise value is generally calculated as equity value + debt - (excess) cash. Ebay has approximately $5 billion more cash than debt, so its EV is less than its market cap.
One way to think about why cash is subtracted is that the acquirer gets to keep it. If I pay $10 billion for 100% of a company's shares, but get to keep the $1 billion in cash on the balance sheet, then the actual price of acquiring the company is only $9 billion (assuming no debt). With enough cash, it is possible to have a negative enterprise value.
EV is usually calculated with the current equity value, as that is what the market "believes" the company is worth. If an acquirer comes along and wants to purchase all of the shares at a premium, you can find an implied EV from the offer price.
One way to think about why cash is subtracted is that the acquirer gets to keep it. If I pay $10 billion for 100% of a company's shares, but get to keep the $1 billion in cash on the balance sheet, then the actual price of acquiring the company is only $9 billion (assuming no debt). With enough cash, it is possible to have a negative enterprise value.
EV is usually calculated with the current equity value, as that is what the market "believes" the company is worth. If an acquirer comes along and wants to purchase all of the shares at a premium, you can find an implied EV from the offer price.