Does the listing offer PayPal as a payment method? That would be the primary reason I can think of that would stop you from going through checkout with PayPal.
Without any more info, the only other possibility I can think of is that it's a listing which requires immediate payment - those purchases can only be processed with an instant PayPal payment (which means you either have to have enough PayPal balance to cover it, or a credit / debit Visa/Master card on file), in which case the item isn't purchased until paid for, and if you don't have an instant funding source, you wouldn't be able to go through checkout.
