Is the seller of used shoes required to inform the buyer that some of the shoes have been repaired, or is he not obligated to do so since they are originally used shoes?

If you suspect that repairing the shoe is disliked by the buyer, or reduces his desire to purchase it, then you must disclose it to him. Otherwise, it is not obligatory. The seller must disclose any defects in the item being sold if the buyer would dislike them, even if the seller is unsure of the buyer's dislike. If he does not disclose it, then it is deception or a lie. However, if it is certain that the buyer will not dislike it, then the seller is not obliged to disclose it.