Sounds as if you are buying via an echeque.
Normally when you pay a seller it is not instant > paypal credit the seller for you and then start the process of uploading your funds from your bank account to pay themselves back again.
HOWEVER they only take the risk of doing that if you have a back up funding source on paypal as well ie debit or credit card.
If you don't have one or the one you have has expired and needs updating then the payment will go as an echeque and paypal will wait till it clears through to them before they credit the seller with your funds.
Advice is voluntary.
Kudos / Solution appreciated.
