Here's how it would actually go>"Hi, I'd like to buy these two boxes for my son">"Very good sir, would you also be interested in purchasing some of our excellent paints for your son?">"No thankyou">"No problem sir, here's your receipt">"Cheers"
This is something that quite annoys me about comics or tv shows that depict wargaming. So often, they just get it completely wrong. I have never, ever, played with someone who actually said "BOOM" and "BANG" when his weapons hit the enemy lines, or made zooming noises with his planes or tanks. Contrary to what some comics would have you believe girls do frequently walk into game stores and many of them are bigger neckbeards than the blokes.
And with regards to that comic, no-one selling Warhammer products, least of all redshirts of all people, is going to not sell a product to someone because they're used in different systems. The most they'd ever do is politely point out that the two aren't compatible, but unless the customer asked them, I honestly can't see it happening.