This excerpt taken from the EBAY 10-K filed Feb 28, 2007.
Funds receivable and funds payable to customers
Funds receivable and payable relate to our Payments segment and arise due to the time taken to clear transactions through external payment networks. When customers fund their account using their bank account or credit card, or withdraw money from their bank account or through a debit card transaction, there is a clearing period before the cash is received or sent by PayPal, usually one to three business days for U.S. transactions, and up to five business days for international transactions. Hence, these funds are treated as a receivable or payable until the cash is settled.