I haven't ever done that, I don't think there's a way.
Honestly though, I've never needed it either. Why would you ever need to sell one potion? You gain money from selling weapons and other equipment mostly, and among the potions there's ones that are definitely better to hoard (health, mana, speed) and ones that are totally useless and are better sold (all the resistance potions). You should never be so short on money that you have to sell 1 healing potion - just go grab some sword of a corpse! :)
You're right. I was just curious if it's possible.