Prices raise for each 1000 bought for 1 gp. Prices drop for each 1000 sold back to the npc for 1 gp. A very long time the buy back price of the npc was half of the actual price. That got changed not too long ago. Nowadays the buy back price is at maximum half of the initial sell price of an item. EA felt that too many were earning too easy and too much money simply by balancing prices between different npcs. So nowadays nearly no one sells such items back to the npcs.
Vendor prices reset on new patches. Besides that there are wandering merchands in Illshenar. Those reset every time when they respawn at an other location. But I dont know, if those offer bottles. (But at least at one of the spawn locations a bank box acces is included. So bulk buying there is possible.) Besides that you might have a try at rarely used vendor locations: Wind Tram/Fel, Papua Tram/Fel, Delucia/Fel, static gypsy camps in Ill, the gargoyle city in Ill. I really doubt, that all of them having prices for bottles rased to 35+gp.
If your really desperate for empty bottles you could buy filled potions from the alchemists and gulp them all.
