It is very tactical element in pricing strategy and you can often use it with products that are of less importance for you as a retailer or eCommerce site owner. 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. Also if you haven’t used the original price before, your price campaign might be illegal, at least in some countries.
This means that you must have sold products before any advertised pricing campaigns can be started. Similarly in cases you lower the advertised price, the discount of that price will be calculated from the advertised pricing instead of the original pricing.
If you use advertised pricing on a category level, you must clearly distinguish what products are discounted. Consequently 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.