The advertised pricing is very tactical element in pricing strategy and often used with products of less importance for the retailer or ecommerce site. The discounted pricing needs to be real, and the list price have to been real and higher earlier on in the same store, just before the campaign starts and advertised price is presented. In some cases you do not need to present the original price in the advertisement, but the original price has to been used. Unless the original price has been used before, the advertised price campaign might be illegal at least in some countries.
This means that the product must have been sold before any advertised pricing campaigns can be started. In cases the advertised price is lowered the discount of that price will be calculated from the advertised pricing instead of the original pricing.
If advertised pricing is used on category level, it must be clearly distinguished what products are discounted. It is often illegal to use advertised prices in way, that lures the customer into thinking that all of the product categories have high discounts, even though when customers visit the stores, they notice that high discount percentages only applies to a small part of the products.