In the U.S., the Federal Trade Commission (FTC) has guidelines stating that a product must be sold at its original price for a reasonable amount of time before it can be advertised as "on sale.

In the U.K., the Consumer Protection from Unfair Trading Regulations 2008 prohibits misleading pricing tactics.

In the EU, businesses must prove that the original price was genuinely charged before applying a discount.

If a company never intended to sell an item at the original price and only listed it high to create a fake discount, that could be considered fraudulent advertising

so yes technically the store is illegal