Article 17, Clause 3 of South Korea's 『Act On the Consumer Protection in Electronic Commerce(commonly known as the Electronic Commerce Act)』 states "If the contents of the goods, etc., are different from what was indicated or advertised, or have been performed differently from the terms of the contract, the consumer may cancel the order, etc., within three months from the date of receiving the goods, or within 30 days from the date he or she knew or could have known such fact."
Article 18, Clauses 1 and 2 of the same law state " Where a consumer has cancelled, etc., an order pursuant to Article 17 (1) or (3), he or she shall return the goods, etc., already delivered: Provided, that this shall not apply where goods, etc., already supplied are services or digital content. A mail order distributor (including a person who has received payment for the goods, etc., or a person who has concluded a contract on a mail order with a consumer; hereafter in paragraphs (2) through (10), the same shall apply) shall make a refund of the price of goods, etc., received, within three business days from any of the following dates."
Article 32, Clause 1 states "Where a business operator does violate Articles 18, the Fair Trade Commission may order him or her to take corrective measures."
Therefore, if you request a return of a defective product and the business operator refuses, you can report this matter to the Fair Trade Commission, a city or provincial mayor, or the head of a district, town, or county (refer to Article 26, Clause 4 of the Electronic Commerce Act). The authority that receives your report will manage the return process of the product.
Furthermore, Article 35 (Prohibition of Contracts Disadvantageous to Consumers) of the same law states "A contract that violates Articles 17 through 19 and is disadvantageous to consumers shall become void."
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