They were in high demand and sold out. The store staff told me that the next restocking date was uncertain. There was nothing I could do about it. It was out of my control.
こんにちは！
ご質問ありがとうございます。
ご質問について、いくつか言い方が考えられますが、例えば、
I wanted to purchase special envelopes for shipping some items I sold through an online auction, and I tried to buy them at a convenience store, but they were in high demand and sold out. The store staff told me that the next restocking date was uncertain. There was nothing I could do about it. It was out of my control.
とすると、『[インターネットオークション](https://eikaiwa.dmm.com/uknow/questions/40233/)で売った商品を発送するため。専用封筒をコンビニで買いたかったけど、大人気で品切れ、次回入荷未定ですと言われた。どうしようもできない、不可抗力です。』となります。
参考になれば幸いです。
high demand 高需要
参考になれば幸いです。
日本 
