半額をチラシなどで唄う際は「50% off」を使う。
質問者様の言いたいことを英語にすると：「My local supermarket discounts some vegetables by 50% after 8PM. So I go there and buy lots of veg. They won't wait for you!」
注意：「早い者勝ち」は普段は「first-come, first-served」ですが、この場合は「They won't wait for you」を使っています。その理由は、「first-come, first-served」は一般的にチケットなど手に入れ難いものに使われます。半額の惣菜そんなにレアな物ではない為、ここで「first-come, first-served」を使うと不自然です。
Sales are a great opportunity to pick up a bargain and there are a few ways of describing how much cheaper sale items are.
You could also say: "All items half price," or
50% Off All Items"
セールは、割引を受けるビックチャンスで、セール商品がどの位安価かをいう2、3の言い方があります。
以下のように言えます：
All items half price.
50% Off All Items.
（半額セールです。）
Both the expressions "half off sale" and "50% off sale" have the same meaning that the item you want to purchase is not full price but half price.
For example
"Look at this shirt! It is normally $50 dollars, but I got it half off!"
"Last year's fashion items are 50% off this week only."
"My friend offered to sell me his guitar for 50% off the price he paid for it."
"Did you hear about the half off price at the store?"
half off saleと、50% off saleの両方の表現は、同じ意味で、購入したいものが正規の値段ではなく、半額ということです。
例：
"Look at this shirt! It is normally $50 dollars, but I got it half off!"
このシャツ見て！いつもは50ドルするけど、半額で買ったの！
"Last year's fashion items are 50% off this week only."
今週のみ、昨年のファッションアイテムは50％オフです。
"My friend offered to sell me his guitar for 50% off the price he paid for it."
友達がギターを購入した金額の半額で、そのギターを売ってくれたんだ。
"Did you hear about the half off price at the store?"
そのお店で半額のことを聞きましたか？
Sometimes stores will have great deals on items and they will have a 50% discount. There are a few expressions we can use to talk about this!
"50% off sale"
This is a very common one and many times we will see a sign outside the store that will say this or something like "50% off everything!"
Two very similar expressions could be:
"Half Off" and "Half Price". These expressions are a little more common in verbal speech but we can sometimes see them on signs as well.
店は商品に対してとてもお得な条件をつけて50パーセント割引をする時があります。これについて話すための表現はいくつかあります。
"50% off sale" (半額セール)
これはとても一般的で、 店の外にこれか”50% off everything!” (すべて半額割引)などと書いてある看板を見かけることがことがよくあります。
類似した2つの表現は”Half Off”と”Half Price”です。
これらの表現は話し言葉として使われることの方が少し多いですが、看板に書いてあるのを見る時もあります。
If you want to say a shop has 50% off their products then you could say they have a '50% off sale' this could also be said as a 'half price sale' they both mean the same thing
半額または5割引きのことを”50%off"と言います。”50％ off sale"（半額セール）と言えば、商品の価格が半額という意味になります。どちらとも同じ意味です。
In American English, when there is a sale of 50% off of the original product, we call it a "half off sale." For example: The store down the street is having a half off sale all week long!
I hope this helps :)
アメリカ英語では「半額セール」は「half off sale」と言います。例えば：
The store down the street is having a half off sale all week long!
（通りを行った所にあるお店で、一週間半額セールをしている）
参考になるといいです :)
The two sentences you see provided above are excellent ways to express to your listener that a certain sale is offering a 50% discount. In the first sentence you will see the term half off. When we are talking about sales and discounts this term means a product is 50% off or half of the original price.
イギリス 
アメリカ合衆国 
