I am shocked that some people are still influenced by online ads. I mentally blocked them and I almost always make informed buying decisions based off of evidence/data.
Even if you do carefully research your purchases, you're influenced by which products you're exposed to, which are reviewed, social media chatter, etc. This is all heavily influenced by marketing efforts which are a lot more subtle than banner ads.
sorry to paint such a bleak picture but this stuff is ubiquitous, both online and offline, and it will eventually and consistently break your mental defenses in a way that you might not even notice.
[1] I use generally and not specifically since on average the ROI is positive but for some advertisers the ROI is negative too.