They don’t seem to be sale items, so what is the deal?
They regen one every 5 or 10 minutes I believe, and they fully heal your current planeswalker.
You carry a max of 5.
With patch v1.3.1, you can carry up to 3 potions.
Each potion takes 30 minutes to refill.
You can use potions before selecting a story level or a quick battle.
You can also use a potion after you win/lose a level.
Potions will fully restore your currently equipped plainswalker.
If you have 0 potions and are missing HP, you will be given the option to buy 1 or 3 potions for crystals 
.
When you have 0 potions you will also see the remaining time until your next potion will refill.
Alternatives to use a potion are
- leveling up for full health
- switching to another PW to regenerate health
- play with a deck that less likely loses health or even heals during a battle