When one goes into the store, one knows the cost of the item that they are purchasing. They know, however, that they must add on a 7% percent sales tax if the purchase is made in NJ, or 6% if the purchase is made in MI. Furthermore, certain states exempt certain goods from the sales tax and even if one resides in MI, for example, one may not know which items are exempt from the sales tax.
Combining the sales tax and the list price would also allow the consumer to know what the final price will be before reaching the register. Many countries already do this and it creates a more efficient system because exact change can be prepared while waiting in line to pay.
For these reasons, it would be better for everyone if the list prices of all products would have the sales tax included.