When a shop assistant asks politely: Can I help you?
The perfect “brush-off-reply” is to say: No thank you, I'm just looking.
That means, rephrased: No thank you, I'll ask you if I need help, at the moment I would like to be left alone.
Of course the person in the shop doesn't insist on assisting you after being told no thank you. That would be rude.
I have been many times at International Magic and I have nothing but good things to say about Barry and Martin. Marco is extremely knowledgable about magic and likes to tease. I've once been there when Jerry Sadowitz was behind the counter and that was an experience I will not forget.
Mongo, I can just say, if you want the sales person to talk to you, don't start the conversation brushing him off by saying that you're just looking. He can't possibly read your mind (or can he?

) and imagine what your field is or your main interest. Shopping in a shop is a two person interaction.