What is the law on selling alcohol if you suspect the purchaser will give it to someone underage?

Most major supermarkets refuse to serve groups of young people unless all of them can provide proof of ID.
When you are genuinely not purchasing for anyone under age, you are made to feel like a criminal.
Are they legally obliged not to sell if they suspect you're buying for under 18s, as in could they be fined etc? I would have thought the person who was doing the buying would be the one punished.
Where does the law stand on this?

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