PHILADELPHIA—Former President Donald Trump announced during a surprise appearance at Philadelphia’s “Sneaker Con” on Feb. 17 that he has launched a Trump-branded sneaker line. The appearance at the nonpolitical event seemed to come out of left field from the Trump campaign. But it quickly became obvious why President Trump was making the abnormal appearance: It was to unveil the merchandise called Trump Sneakers, which are available for preorder online. A website launched concurrently with his appearance shows that President Trump’s new sneaker line comes in three variants. A special limited edition sneaker, “The Never Surrender High-Top,” was already sold out...