Can an employee be liable for an underage alcohol sale if he/she wasn't properly trained to handle such sale?

For example, I have been informed (spoken and written) by my employer to check I.D. for individuals who appear to be under 30 and that the I.D. must valid (age, expiration date, accurate picture). What if say, that I was never informed about "second party" sales but took part in it or any other violations of my state's alcohol law, would I be liable? A scenario: Two people walked in together with some groceries and among that contained a bottle of wine. First person hands me his license, he is of age. I accept it and enter his information for the wine sale, I assume he'll pay. However, when it is time to pay the second person hands me cash and I say (before taking the money) I need to see I.D. He is unable to provide it to me and I tell him I cannot accept his payment. His friend (the person I took I.D. from) says "I'll pay for it." I allow him to swipe his card. Is this a second party sale? Will I be liable for the sale if say after the 1st guy swipes his card, the second person hands him the cash he was going to use to pay for the transaction and if it was later discovered that he was underage? Should I have stopped the sale completely?