Sell them to Oswald Fiddlebender in the broken ship (the NE section of Kuldahar, go straight east from Conlan's Smithy).

Have you not encountered Oswald before, or did you just not know that he buys potions?

If you've missed Oswald on previous occasions, here's a couple tips on him:

MINOR, MINOR, SPOILER
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1) He buys and sells potions. I usually don't need to buy any, but if you do (playing without a priest, or with a small party, or just plain use lots of potions), I think his are the cheapest.

2) Pickpocket him like crazy! He's got some great stuff, especially considering how early in the game you get it. iirc, he's got a necklace, a ring, and three potions.

3) I've heard he's got a few more items, should you choose to kill him. I never have, because it's bad roleplay (he's harmless, after all). Also, if you kill him, certain townsmen become very unhappy with you, and the reputation loss is enough to make paladins and rangers "fallen." It may even prevent continuance of the game, e.g. if Arundel won't speak to you.