double-spend attack is simpler. The only requirement is to mine simultaneously two versions of the next block: one containing the transaction from you to the seller (who does not wait for N confirmations), and the other with transaction to you. If you find both blocks, then you send the first to the seller (and get your goods), and the second — to the rest participants. There is high probability that the second version of the blockchain will be continued, and you will get your money back.