I bought a recipe from Arcadia in Whiterun. Saved, quit the game and when I came back to it, she had another potion recipe for me right away. I tested it out. Saved, quit the game (not to the main menu), booted the game back up and loaded my save, the same new recipe was there plus another one. Eventually I repeated this and there was 3 recipes. I know Bethesda games have a back habit of keeping events/scripts lingering in the background even if you quit your session and load up another save while you're still in-game, such as enemy encounters, physic behavior, music, merchant gold staying low when they buy stuff from you, when it was just refreshed but still low when you loaded a save just before making any transactions, etc...

You can save, buy stuff, save, punch the vendor, then load the first save to reset vendor inventories/money.