To buy up (large quantities) — to purchase and hoard goods, often reducing supply or manipulating availability/prices.
パニックで人々がトイレットペーパーを買い占めた。
In the panic, people bought up all the toilet paper.
転売目的で商品を買い占める業者が問題になっている。
Resellers who buy up items for resale have become a problem.
新製品が出ると一部の人が買い占めてしまうことがある。
When a new product comes out, some people sometimes buy it up.