A seller accepting 0-confirm sales is vulnerable to a Finney attack but given the staggering amount of hashing power required to even attempt that nobody is going to spend $80,000 to build a hashing farm to double spend a 1BTC copy of angry birds.
No, but they will transfer hundreds of thousands of LTC into exchanges, sell it and withdraw, then rewrite the chain. But true, it's not worth for small business amounts to wait hours for lot of confirmations.