We have to get RMA's from many of our vendors from time to time. Don't know which side of the equation you will be working, but it can be viewed as a purchase order system for goods post sale. Just as a purchase order is designed to control the purchase process, a RMA system is designed to control the process of merchandise being returned post sale. Many companies want to add this layer to better handle defective or damaged goods, as-well-as returns for no good reason. Sometimes restock fees are a part of the mix and can cause complex sales tax considerations.
Best of luck to you,