1. What does it mean?
풀박 shortens full box. In secondhand trading it usually means the product comes with the original packaging and the accessories or inserts expected in the set.
Also written or said as pulbak
2. The nuance
It does not mean unused, sealed, or necessarily authentic. Sellers may interpret the complete set differently, so the actual contents should be listed or photographed.
3. Examples
“풀박이라고 해서 설명서도 있는지 확인했어.”
Since it was advertised as a full set, I checked whether the manual was included.
“본체는 깨끗하지만 상자가 없어서 풀박은 아니야.”
The device is clean, but without the box it isn't a full-box set.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.