Unconfirmed transactions are many orders of magnitude safer than "taking someone's word for it."  The transaction wouldn't have propagated through the network if it wasn't valid, and it will be included in the next block regardless of whether the sender wants to revoke it.  If the seller can see the transaction on their screen, then it's currently not possible to reverse it without a multi-milllion dollar double-spend attack or something very creative. 

I believe, the risk of chargebacks on credit cards are much higher than the risk of an unconfirmed-but-valid transaction not eventually entering the blockchain.