I would have never thought to call that the soul market. I always thought of that as the inventory screen.
Accidentally selling stones is not an uncommon problem. This is why I have suggested a separate location to sell them via a button in the soul mart. I've nearly done it several times now, and I know several people who have actually done it now. You can file a report, but based on what I hear, they won't award it.
I would also like it if five stars can be sold for gold or market tokens, as sometimes I have had to sell a couple soulstones for incomplete heroes to make a rank reward. But it is what it is. The most important part is that they either make a separate location to sell them, or put a breaking line between five star heroes, and incomplete heroes to prevent future issues with this.
In the meantime, all I can tell you is to be deliberate in your selling. Don't try to do it fast. Give yourself a moment to register whether or not you are on the right hero before hitting sell.
It's a tough lesson to learn when you lose almost an entire fifth evolutions worth of stones.