I'm really tired of buying each potion and move it to the right position again and again. I use a lot of potions. If someone can make a mod of stacking potions and/or fast buy/buy 10 at once or something like that, I'd be really grateful for that.

Below the merchant's inventory you have a bunch of icons (identify, repair, etc...). One of them, once clicked, allows you to buy items without having to manually move them. The only downside is that they'll be automatically placed in the 1st available space of your inventory, so you may still have to move them afterward.

You can do what charbon said (it's icon most the to the left that buys) and download a macro application (or in case your mouse supports it) then add that if you hold Ctrl + Mouse1 you press Mouse1 10 times really quickly that way you only have to press 3-4 times and you should have enough.

As always, back up saves before modding or doing anything like I said.

Thank you very much for your answers. It help and save me a lot of time.