When you go shopping, it is very important that you know whether or not an item is on sale. Sure, there are signs, stickers, and ads that say an item is on sale, but is it? Can you pick out a sale if it is not advertised?
For example, StoreA is having a "sale" on bread 2/$4. However, regular/informed shoppers know that bread is regularly $1.75 each, not $2 each. StoreA is counting on some customers not knowing or caring what the actual price of bread should be. StoreA might also change the price of bread to $1.00 one day without advertising it. A smart shopper with a price list will know to buy bread that day and possibly even stock up for the future when bread is back to $1.75.
Price lists (stock up lists) will help you know when items are on sale and when you should buy an item. Items on the list should be items you buy/use regularly and how much you want to pay after discounts and coupons. Personally, I know that I want to buy bananas at about .33/lb or 3lb/$1, because that is a good price in my area. When bananas are .59/lb, I DO NOT buy bananas! Now, I know that is only .26 difference, but small amounts like this on a cartload of items can add up fast.
Price lists are unique for each shopper. In your area, .59/lb for bananas may be a good price. So, on your list you would put, "bananas .59/lb". No matter the price, it should be a price you are willing to pay. I can get makeup for free or almost free, but since I don't use it, I'm not willing to buy it at ANY price. No matter the price, it's not a good deal unless you will use the item!
**I have to say that price lists are also good for keeping track of your progress using coupons. When I first started using coupons, I was willing to pay $2/box for cereal. Now that I know more about sales and coupons, I'm only willing to pay .50-.75/box for cereal.**