Hi, this is JOA GG, and we are happy to assist you.
As for the trading of Kpop artist photocards in Korea, the biggest platform is Bunjang www.bunjang.co.kr (they have mobile app too but we are not sure if you can download it in your country as well or not.)
To search for the photocards, you can enter this keywords and have a look
photocard: 포토카드 (or 포카)
BTS: 방탄소년단 (or 방탄)
Here you can see the price list of the current available items for '정국 포토카드 Jungkook photocard'
If anybody else knows any other platform than Bunjang, please leave a comment below and let us know!