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buy & sell occurs at an exchange, which is instant as long as the exchange does it instantly. The exchange is keeping tracking of which account inside their exchange has what stuff. So it is just the exchange writing its records to database.

This has nothing to do with how long a bitcoin transaction (sending bitcoin from address a to address b) takes. bitcoin transaction occur in bitcoin network. The buy & sell is just internal to an exchange (eg. mt. gox) and occur outside of bitcoin network.