Nobody has said anything about selling.
We are all talking about what you get instead of the potion if you have no space to take any more of that one.
You can always hold all potions & books, even if you can't yet use them, so that will be the same, then once you have 5 of them, you will get 10 gems instead (which is standard for all potions).
Thanks TerMinus Prime for the sig pic