Fui al centro comercial vi unos tenis i los compre que compre?

 sport shoes, trainers, sneakers, runners, etc.

The direct object of the verb "comprar" is "tenis". We can replace it for a direct object pronoun. Since tenis is a masculine and plural noun, this pronoun must agree in gender and number. "Los". As the verb is conjugated, the pronoun must be inmediately before the conjugated verb: los compré. If the verb would be in the infinitive form, we could add the pronoun to the verb. Example: Fui al centro comercial a comprarlos.

Explanation: The sport shoes have gone by a variety of names, depending on geography. The term "sneakers" is most commonly used in the United States The British English equivalent of "sneaker" in its modern form is "trainer".