...So, what do you think, does Haggler count here?
Argument for: Play Haggler, Silver, Potion. Buy a Golem. Use Haggler's power to gain an Apothecary. Since Haggler's power is on-buy, the Apothecary hits your discard pile before the Golem does, so strictly speaking it is Haggler that has just gained you your first potion-costing card.
Argument against: In order for Haggler to be able to gain you a potion-costing card, you have to play a Potion also on the same turn. In that sense it is Potion, not Haggler, that is actually enabling you to gain a potion-costing card. Consider this characterization of Ambassador, Jester, Potion, and Thief: "It is necessary for at least one of these to be played in order for someone to gain their first potion-costing card on the current turn." Haggler does not add to that list.