1. What does it mean?
쟁이다 means stacking or storing things; in shopping talk, 쟁여 두다 is stocking up on a favorite product or supplies. It often appears when a sale prompts someone to buy extras.
Also written or said as jaengida
2. The nuance
It is not necessarily hoarding or a huge purchase. The same verb can appear in other contexts, so the shopping sense depends on what is being stored.
3. Examples
“자주 쓰는 공책을 몇 권 쟁여 뒀어.”
I stocked up on a few of the notebooks I use often.
“세일이어도 보관할 곳이 없으면 쟁이지 않아.”
Even on sale, I don't stock up if I have nowhere to keep it.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.