That is a risky method due to invalidating a Tx already in a block. Consider if a dbl-spender mines his own block with a Tx that dbl-spends coins, invalidating a zero-conf Tx he sent a Merchant A. He then immediately pays Merchant B from the dbl-spent coin who accepts zero-conf Tx (presumably because of their trust in this proposal). If he pays Merchant B before he hears about Merchant A's invalidated Tx, then Merchant B will have their zero-conf Tx invalidated and the dbl-spender gets away with effectively dbl-spending coins (got same coins accepted by two merchants).
No, that can not happen if merchant B does a proper job. Merchants always have to wait and listen for a couple of seconds for previous TX and then broadcast the TX afterwards. Merchant B could even sell the double spending TX to a miner without giving away the goods.
This also sets up the possibility for extortion, where a buyer threatens to dbl-spend coins (making the merchant lose the coins) unless the merchant does something.
I don't think extortion is a problem at all for the targeted kinds of goods.