Trusting an anonymous redditor because they have lots of "karma" is extremely foolish. For one, they probably got those internet points by reposting other people's cat pictures and now you're in a conversation with them about something that has nothing to do with the subject of stealing cat photos, so why should the internet points they earned doing that count for anything?
Secondly, it's easy to farm up these internet points and sell the account to somebody else who's keen on exploiting the tendency of people like you to think that the internet points confer trustworthiness.
All of these trust systems and platforms are a centralized target for the behavior you describe. It makes it easier for those with bad intentions to rinse and repeat. Naive users may be left behind by those who have optimized their strategy.
Strawman. I never said anything about karma. A user's entire post and comment history is one click away during a discussion. That can be skimmed to build a decent picture quickly. That too can be faked, but the bar is much higher.
Secondly, it's easy to farm up these internet points and sell the account to somebody else who's keen on exploiting the tendency of people like you to think that the internet points confer trustworthiness.