Hold control key while pressing the left mouse button on one card you do not own.
Then, do the same for another card, and another, until satisfied.
Click to purchase, then click to confirm. All cards will be bought and added to your collection.

or

(be careful with this)
Hold control key while pressing the a key. This will select every single card shown based on your filters or search. Only purchasable cards can be bought this way, the other cards (ones you own and SEC) will still be selected, but not bought. Click to purchase, then click to confirm. All purchasable cards will be bought and added to your collection.

in both cases, the confirmation pop-up tells you how many cards and how many credits for them, then click 'get them nude'.