Hi, Vermont, This is how I read the passage: If you have anything up your sleeve (=some hidden/secret ability) you should show it now. If you can borrown someone's hat in the audience and make customers for an idle (unnecessary/unneeded) stock of shoes come out of it (a reference to a magician taking a rabbit out of his hat; compares your abilities with a magician's ones) you'd better spiel (=advertise, speak persuadingly for the purpose of sale). The passage shows that the boys have laid in enough footwear to last them ten years (=have stored away a lot of shoes), so if you have the knack for persuading them to buy more shoes, you should go into the advertising business.
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