Are you sinful if you return a poor-quality piece of clothing to its place after trying it on in a store, without informing the salesperson, knowing that the manufacturing defect is the reason and not intentional damage?

If the method of trying on clothes does not exceed the usual limit and does not involve any transgression or damage, then there is no blame on the questioner for what she mentioned, because custom has established this practice, and shop owners prepare places for that. There is no liability on the customer unless he or she is negligent or transgresses. As for returning to the seller to inform him of what happened, that is better, by way of sincere advice, based on the Prophet's (peace be upon him) saying: "Religion is sincere advice."