You never need funds in your paypal account to buy items as paypal will take direct from your funding source attached to your paypal account.
If you do however have funds in your balance ie from a buyers payment then paypal will use those first as long as the funds are not pending or processing and are in the same currency as the person you are paying accepts.
To use those funds however you need a back up credit card linked to your paypal account as well.
If you have only a bank account linked then you "normally" won't be able to use it unless again you have a back up credit card linked to paypal as well.
Sometimes however paypal will let you use the bank account without a back up card but the payment will go as an echeque and take a few days to clear through to the seller.
If you pay via a debit / credit card (or bank account with a card as back up) then paypal will credit your seller instantly for you and then paypal will take the wait for your funds to transfer from your funding source. This is to enable you to get your item more quickly.
If there are no cleared funds then paypal will take from any bank account if you have one linked BUT you can swap to a card instead of a bank account by clicking on "other funding sources" DURING the payment process to select a card on a transaction by transaction basis.
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