Dont think theres anyway ingame. If you sell potions (or whatever) to merchants, then naturally they are guna be able to sell them straight back.
Howevery if your making your own potion, obviously they arent guna be the standard potions sold (eg, its impossible to make "p_restore_health_e" potions Edit: without cheating
)
The only way to actually increase the number of potions sold by a merchant is to edit the number they have in the CS.
Eg, if you want nalcarya to sell 20 standard restore health potions, you would change the number to -20.
HOWEVER since im pretty sure she has (-) negative values on all her stock, all you need to do is buy all she has for sale (eg: 5), exit the dialogue, and then go to barter with her again and she'll immediately have more anyway. so dont waste your time cs'ing it.
Think this answers your querry.