The content related to this question is found in the Korea Customs Service FAQ, so I am quoting that information below and providing an answer based on it.
◎ The customs value of imported goods shall, first and foremost, be the transaction price adjusted by adding the relevant components listed in each subparagraph of Article 30(1) of the Customs Act (hereinafter referred to as the “Act”) to the “price actually paid or to be paid” (hereinafter referred to as the ‘actual price paid’) by the buyer for the goods “sold for export to Korea” (hereinafter referred to as ‘export sale’). - ‘Sale for export to Korea’ refers to a ‘sale’ that occurs immediately before the goods arrive in Korea as a transaction involving the actual international movement of goods; B/L transfer transactions that occur after the goods have arrived in a bonded area in Korea do not fall under this category.
◎ Therefore, the transaction price of the ‘sale that occurred immediately before arrival in Korea’ must be determined as the customs value of the goods. If the person liable for tax payment is unable to determine the aforementioned transaction price at the time of the import declaration, the customs value shall be determined by sequentially applying Articles 31 through 35 of the Customs Act.
In other words, the transaction amount reflecting the transferor's margin through a B/L transfer transaction cannot be the actual customs value. Since the customs value is determined based on the sale price established before arrival in Korea, the amount stated on the commercial invoice formed by the transferor’s transaction with a foreign party must be considered the customs value. If such an amount cannot be clearly determined, it means the customs value shall be determined by sequentially applying Articles 31 through 35 as prescribed in the Customs Act.
[This content regarding export and import clearance regulations and their interpretations is based on the customs and trade laws of the Republic of Korea.]