Potions are generally for sale through Alchemists, Scrolls through Clerics & Mages.
Other magic items like weapons, armors, etc are generally NOT for sale. anyone who has those items do not wish to part with them.
Scrolls can be made with the help of the NPC Spell Costs table on pg 113 of the 2E DMG.
As for potions, I use the tables on page 135 of the DMG and since it doesn’t actually have a GP values, I multiply the EXP value by 20. That is the price the PCs have to pay for each potion to be made by an Alchemist. But there should be some potions that a PC SHOULD NOT be allowed to purchase because of how powerful they are. eg. Dragon Control, Human Control, Etc.
I hope this helps, Good Luck