You ask where is fun in buying potions ?
Well first and foremost you should be able to buy only basic potions from most sellers and some of the more advanced potions only from the most famous alchemists like Kalkstein from Witcher 1 is.The most advanced should still be eiher extremly expensive or hard to find merchant who would sell it.
They will be more expensive than gathering ingrediants but more easier.Also you could get discounts if you do some services to merchants.
And let me counter your question (to those who asked it) where is fun in buying potions : Where is fun in Geralt spitting in bottles and it transforming into any potion no matter how complex it is to make or how rare ingrediants are to be found ONLY because he made one of it 3 in game months ago??
Well, you seem to have missed the point.
As I said, I like creating the potions, there's the fun for me. I could buy of course, but rarely.
As for the merchants that sell them, ideas were mentioned just above, probably you haven't gotten to it yet.
That would be a nice compromise to have both parties happy.