Unless you're using an exchange (admittedly most are) for your trading then it's still your machine telling the network it'd like to do a transaction - somebody

Unless you're using an exchange (admittedly most are) for your trading then it's still your machine telling the network it'd like to do a transaction - somebody is selected via proof of work to confirm it - and then you're on your way. Only you can start the transaction, and it can only be received by the one you addressed it to. The way credit/debit cards work is far different IMO.