Okay. Here's the run down.
When you get your heroes to 5 star, they stay that way.
All stones are stored in your inventory, under the stones tab.
Open the stones tab, and any hero that is 5 star will be at the top of the list.
You sell them the same way that you sell anything else in your inventory, except instead of giving you gold, you get 100 soul mart tokens each from selling them.