Selling poisons even using them is not an evil act. Just the same as using your sword. Some cultures may look down on, make them illegal or the honorable may deem them dishonorable but no it is the intent behind the use that is evil. If you use a lethal poison to commit a murder in cold blood because the act itself is already evil. Some use poisons as an equalizer to someone's skill or might for their survival or to subdue an enemy you don't want to kill or even in defense. Question should be are they illegal and the paladin I upholding the law. If they aren't then they don't have any ground to stand on other than the paladin thinks them dishonorable.
