The roots of this plant were still pretty pliable so I was able to manipulate the soil enough to loosen most of the roots and spread them out into the hole I made in the soil that I intended to plant in. But when I buy plants midseason, I often encounter a stubborn network of roots that I cannot pull apart with just my hands. Depending on how badly I want the plant and how good the price is, I will sometimes buy it anyway (I might also ask for an additional markdown, a request that is often granted). For those gardeners who are faint of heart, I would recommend that you stop reading here. Next, I get the plant home, soak it, and then go to town with the scissors. I make four vertical cuts with my Joyce Chen scissors from the base of the plant, halfway up to the crown and then just start pulling it apart. Many roots get torn in the process. But it has been my experience that enough of the root system remains intact so that when I plant it, it is able to establish itself quite nicely.
What I'm saying is when you see a root-bound plant, make an informed decision. Just because it's severely root-bound doesn't necessarily mean it can not eventually flourish. You can choose to leave it on the shelf, or if you love it and must have it, you can try my approach.
Bookshelf: Container Gardening Books