Coinbase may have separate limits for the different account funding sources listed below.
Bank transfers (ACH, SEPA, GBP, Bank wire)
Instant purchases via bank transfers, enabled by account history and a backup credit card (available to select users in the US only)
Cash balance stored value (USD, CAD, EUR, GBP, etc.)
Instant buys (credit card/debit card, where available)
You can add funding sources on the payment methods page. You can also fund your Coinbase account with a wire transfer (not available in all regions) for up to the buy limit listed on your verifications page, without linking a bank account to your Coinbase account.