Each customer can be set to a specific pricing level, so when you add an item to a transaction it uses the value in the pricing level set for the currently selected customer.   If a customer does NOT have a pricing level selected, or you are using the Rapid Sale customer, the price for the items on the transactions will default to the "Sale Price" value.  

The pricing level for customers is set at the time the customer is created, and you can select the default pricing level for the system via "Options - Company Setup / Customization - Company Information and Settings".  It's about half way down the left had column of the defaults tab.

You can also move customers from one pricing level to another via the "Customer - Price Level Maintenance" function.

I would suggest updating your pricing to set all of the items to have the price that you sell the item at in the "Sale Price" field and then update all of your customers to the "Sale Price" level.  The other option is to set the company setting to default the pricing to the list price and then updating all of the customers to the list price level.  The downside to that second option is that you will NOT be able to use the Rapid Sale function as it will still default to the sale price.