Playing 1E we occasionally had a friendly shopkeep who had a magic item or three. Usually armor or weapons. But it was decided by the GM beforehand, so what he had, if anything, was all that you could buy. Later, but still 1E, we changed that to a percentile roll. The more powerful the item, the lower the percentage chance that one would be available. (More than one time a character went looking for something in particular and ended up buying something else because that was what was available.) Potions were usually bought from the clergy. (Mostly healing potions, of course.) After all, why wouldn't they sell them to people of the right alignment? (Unless there was great need of them in town for some reason. In which case our Cleric(s) would volunteer to lend a hand with the healing...)