"I went shopping yesterday" is just fine. Good work there. ;)
When we talk about a store hosting a sale, we say that the store is "having" a sale. For example, "Macy's had a big sale last weekend for Black Friday."
Or, more specifically, "Macy's has a sale on shoes today."
If we are talking about an item being sold more inexpensively, then that item is "on sale". So, "I am going to buy those shoes, since they are on sale."
In your example, I'd say "The shop where you used to go was having a sale."
As for the discount, it'd be more common to say "Everything is 50% off."
It is technically correct to say, "Everything is discounted by 50%", but this is awkward.
Hope that helps!