I'd say its about 50/50 for me encountering testimonials that come accross as clearly fake. These are most easy to spot where the seller is hyper local, but the reviews are from obviously not local-sounding names. But then I do come accross testimonials that appear legit, and they do help with my buying decisions.

I would often rely on a Google review of the seller to determine whether or not it's an outright scam, and for hobby related stuff I might rely on forums where fellow forum goers might recommend a specific product/service.

So basically for me personally I would prefer independent 3rd party ratings/reviews/recommendations, but at this point I'm even a bit allergic to things like trustpilot, as I fear for incentives that are profit aligned rather than customer trust aligned.