I don't know how it is where where you live, but here there are two possibilities I can think of:

- When I buy an item I still have to click a "check out" link to enter my address and actually pay for the item. I could take days after buying the item to click that link. - Some sellers might not accept PayPal, instead after I check out I get the sellers bank information and have to manually wire the money. I could take days after checking out to actually perform the money transfer.