They were selling cockroaches as food for reptiles.
They were selling cockroaches that customers can feed to their reptiles.
They were selling reptile livefood such as cockroaches.
ご質問どうもありがとうございます。
様々な表現があると思いますが、いくつか自然な表現を紹介します。
1. They were selling cockroaches as food for reptiles.
（逐語訳）彼らは(=店の人間)、爬虫類の餌としてゴキブリ売っていました。
2. They were selling cockroaches that customers can feed to their reptiles.
（逐語訳）彼らは、お客さんが、お客さんの爬虫類に餌として上げられるゴキブリを売っていました。
3. They were selling reptile livefood such as cockroaches.
（逐語訳）ゴキブリのような、「生き物餌」を売っていました。
お好みに合わせて使い分けてみてください。
ご参考にしていただければ幸いです。
Cockroaches were being sold as food for the reptiles
The pet shop was selling cockroaches as food for the reptiles!
Cockroaches = ゴキブリ
As food = 食べ物として
Reptiles = 爬虫類
Sell = 売る
Cockroaches were being sold as food for the reptiles = ゴキブリは爬虫類の食べ物として売ってた
The pet shop was selling cockroaches as food for the reptiles! = ペットショップはゴキブリを爬虫類の食べ物として売ってた！
「ね、聞いて！〜、信じられないでしょ！」と言いたい時は :
“You’ll never believe it! They were selling.....”
や又は
“Can you believe they were selling ....?”
ドイツ 
日本 
