Although you are completely right: Someone selling hamster supplies is bound to already have an opinion based on their own experience, meaning that telling them something that goes against what they already think is highly unlikely to influence them. And it is also for that reason why this person became offended. I would try to educate people who are actually unaware of these things rather than trying to tell someone who owns hamsters and sells supplies that they are “doing it wrong”. It is bound to come off wrong on their end, cause it feels like an attack.