Good question. I have big groups of them, but I always transplant mine in the spring when I can see them, with the leaves of course sticking above the ground. I just went out and poked around gently where I know they are, and couldn't find any sign of them (I didn't go more than half an inch deep) but I know they have a root kind of like a carrot. I think I would spit the difference, and plant the crowns aboout an inch deep. You could try some at ground level and some deeper, see what happens. They are wildflowers, they may just pull themselves where they need to be.

It is the most beautiful spring ephemeral, isn't it? If you plant Priumula veris and Primula vulgaris nearby they will bloom at the same time and complement each other very nicely. I let my Mertensia set seed and spread on its own. Now it has spread all over my garden. It is a wonderful sight in spring.

I would transplant in the fall. The roots might get damaged if they stayed in the pots all winter.

Don't know about buying dry root Virginia bluebells, but the ones here (planted in spring) are long-term survivors in a sunny mixed perennial bed. As mad_gallica said, they are spring ephemerals and bloom here in May when the sun is not so high in the sky. Our summers can be quite hot and humid, but the Virginia bluebells are gone by then. I presume that they don't even need the protection from the summer sun which other spring blooming (non-ephemeral) perennials, like primulas, get underneath the taller sun-loving perennials of summer. May 25, last year.