I didn't really have a problem with it. No potion selling required. Biggest problem is probably figuring out the walkthrough more than it is figuring out the game. The very, Very first step is hiding at the dining room at dinner time(evening), after that you just have to listen at upstairs door(no more dinner required, but it happens After dinner time), and after That, ANY next day, hide in the hidden office space in the day time When Giron Is In The Office(just ignore the time, it can be day, afternoon, ANY time he is in the office and Dave is in His room). Conversation will trigger. Then you just repeat Door listening and Office a few times. The First time they talk about the wine, you probably have to go in DAVE's ROOM at night and check out the wine bottle, after that it probably doesn't matter.
I don't know, maybe there is some conflict with step 2 and 3 also if you're there, because I never even got to those steps yet.