Anyone knows what the poison potions are for? I can't seem to "use" them.

And you shouldn't ;)
They are just 'traps' for those that drink / use potions without identifying first - you can use any potion then and Poison will give your party character, unsurprisingly, 'poisoned' status.

You can sell them to the shop near the palace.

Ah. And i was thinking they might serve a purpose for eg. - for the rest of the party to commit suicide halfway through a fight should a party member fall, in order to cut a battle short and reload. Oh well. Thanks, Thiev.

Well, all items have fixed price. You can see which potions are poisons without identifying them in a shop (they're worth 75 gp). That way you can keep them 'usable'. But IMHO, it isn't worth the hassle ;)