it should also be noted that if you are buying something very expensive with bitcoin you can make the transaction offline and it will be instantaneous. You can do this with paper wallets/physical bitcoins. So like if you are going to buy a car you dont have to wait an hour at the dealership. You just load up a barrier bond with the full amount of the car before you leave the house. When you get there if you were able to talk the salesman down than you would negotiate to have those bitcoins refunded as a rebate. The car salesman would simply look up the public key in the blockchain and make sure it had 6 confirmations and then forward the funds to his own wallet.
I'm going to need more details to understand what you are trying to say.
I don't see how the scenario you are describing is any different security-wise than an online transaction at the time of purchase.
I do this sometimes when i sell on local bitcoins, i just hand the customer a cd with a pre loaded private key saved in a text file. Then instead of sending the transaction over the network and waiting for 10+ minutes for a confirmation, he can just load the private key into a temporary wallet and then send the funds over the network to his own wallet. This makes the transaction take only 1 minute instead of 10 and because the customer himself is the one who makes the transaction and since he doesnt have to worry about himself defrauding himself he has no risk of being defrauded. The one downside is that i have to load the bitcoins onto the private key more than 1 hour before i make the exchange and the price of bitcoin can fluctuate a little in that time but if products prices were denominated in
BTC that wouldn't be an issue. (which was assumed in the earlier example of buying a car with btc) In such a world you might for expensive purchases carry BTC cash in your wallet.