Are you sure this actually works? I've never been able to get a vendor to restock with a level difference like that. It seems to check character level on barter, and only restocks if it's greater than current item level on the vendor. Having a companion check and buy the stat books at level 12 and immediately checking with lvl 11 player character (having not previously purchased any stat books at level 11) does not result in a respawn, at least on the current steam version. Are you using GOG / an older patch?
It works, at least for my case.
Lets say the four members are all at lv 10 and have just bought the books, so now the Teller have no books.
Given Madora/Jahan have a significant EXP gap with the hunters that they can level up to lv11 first, and now Madora/Jahan are at lv 11 and hunters at lv 10. Go to the Teller. If you talk to the Teller using the hunters you will of course see no books because she has not restocked (still at lv 10), but if you use Madora/Jahan to talk to the Teller and enter the shop she will restock (becaise M/J advanced from lv 10 to lv 11). After she has restocked you can use whoever has the most bargaining power to buy stuff. And maybe you should use your hunters (lv 10) to talk to the Teller before leaving to make sure the last contact is still at lv 10.
So when your hunters has leveled up to 11. Go visit the Teller and talk to her using the hunters (advanced from lv 10 to 11), there she will restock again.
Madora and Jahan begin with at least lv 3; min XP of 8000. You should already know that they will level with the player (EXP and lv) if the hunters have EXP >8000 at the moment they join.
I skipped the tutorial dungeon and other exp stuff (Shell, waypoint, etc.) By the time I entered Cyseal My hunters were at lv 2 (EXP 3000). You will get the harbour waypoint (90 XP), and take the long route to get around the fish thief. So you can get Madora/Jahan with an EXP gap of around 5000.
You can actually create two significant EXP gap exploiting the fact that M/J only concerns the EXP of the hunters.
Let's say you recruit Jahan where your hunters are at EXP 3000
(so that EXP H: 3000, J:8000).
Go earn some EXP, e.g. 2500.
Then it will be H: 5500, J: 10500.
If you go recruit Madora this time she will still begin with 8000 because the hunters are below 8000.
Then you will have H:5500, M:8000, J:10500
At low levels (<lv12) where EXP earn per enemy is still rather low, you can manage levelling of party members carefully and restock 3 times per level.
