Where I get truly confused is when the customer's accomplice hands me the cash or swipes, and says, "this is their money, this is their card, I'm not really buying it". Am I still required to card them if they claim they're not paying for it?
After arguing, what if the friend just hands the adult the cash/card in front of me? It seems sketchy but i could be wrong.
This happened to me once, and I kept insisting for an ID, but they started getting angry at me, so I backed down because I didn't feel like arguing and there was line I had to shorten.
But now that my supervisor made us sign a notice saying we understood the consequences of selling alcohol to a minor, I want to do the right thing next time. Am I wrong to check the friend's ID?
Yes, you are still required to card them. If they refuse to show their ID, you need to deny the entire sale. Check the liquor sales laws for your area and make sure, but where I live, if a group of people is purchasing alcohol every member of the group needs to be carded. It's illegal to sell alcohol to a minor and there are many and serious consequences for it, so next time, don't let a pushy customer or a line scare you into doing the wrong thing. Those customers are the ones holding everything up, not you. If you run into trouble, call your manager over rather than making the sale.
Is he handing you the money, is he holding the alcohol or asking for the alcohol? Then he is buying it, period. You are not going to get in trouble if a cop overhears it, he'd correct you because you are being duped. But next time, say: "this card is clearly not you. I cannot sell you alcohol until I see proof of your ID as required by state law." If he says that he is not buying the alcohol and that he is only carrying it for a friend, say "Then they should come here and purchase it or let someone with proper ID that shows that they are at least 21 years of age, by law I cannot do it any other way. If I were to do that, I would lose my job and fined."
Different states and countries have different rules regarding selling alcohol. But yes, YOU need to be very smart about selling alcohol. If the customer does not show his/her ID when asked, you have every right to end the sale and call the manager over. If someone comes in who is of age and can buy alcohol, and buys it but gives it to someone else, that sale was between you and the first person, not the second. In the scenario you gave, I would have said, "if you cannot show me ID, then I cannot sell to you." You are liable, so be smart, and keep that in mind. You don't want to go to jail for being timid or because the customer bullied you into selling to them.