You're ignoring the informational role of retail brands. Consider toasters. I'm not a toaster expert, but when I buy a toaster at Kmart I can reasonably assume it won't electrocute me or catch fire in normal operation. I might only buy a toaster once every decade or so, but Kmart buys and sells tens of thousands at a time - it's definitely worth their while to hire an expert who looks very closely at what they are selling and responds appropriately to news about risks. Electrocuting your customer or setting their kitchen on fire is bad for business, so in order to minimize their legal/insurance expenses Kmart is quite likely to adopt such strategies as:
(1) sell mostly trustworthy brand names
(2) require 3rd-party certifications such as the UL seal in cases where those are worth what they cost
(3) run their own tests in-house as appropriate
(4) stop selling and remove from store shelves any brand that turns out to do notable harm.
Kmart decides what to sell attempting to achieve a particular balance of safety and quality, so I don't need to be informed on every specific brand I might find there - I just need to learn and know that Kmart either is or isn't generally trustworthy. If they are trustworthy, I can indeed "just grab something off the shelf and expect it to be safe and effective." (The same is true for specific name-brand product lines such as Craftsman or GE. In fact, the more a brand advertises, the safer it is to try that brand. Spending money on, say, a SuperBowl ad is equivalent to posting a performance bond - it involves spending a lot of money in advance that you can only get ROI from if the company expects to be around for a long time.)
Turning to supplements, I can be nearly certain that they are safe by that exact same logic - brands that clearly aren't safe tend to disappear pretty quickly from store shelves. (Because poisoning your customer is bad for business.)
I can't necessarily be certain supplements are effective and the FDA is, oddly enough, in part to blame for that. The problem is that supplements are not legally allowed to make health claims. They don't say on the bottle what the pill is for or exactly what outcome is expected. It's tricky for something like Consumer Reports to test whether health claims made for a product are accurate when no health claims are being made. So that one's tricky. It's possible we'd do better there with less regulation. A true free-for-all environment would let companies run ads and print packaging not only claiming their product had certain benefits but also (this is huge) comparing their product to competitors and pointing out that theirs works best. If a few large supplement companies are selling Resveratrol pills and the one called "Best" cuts corners using excess filler or a degraded supply, it's hard to think of anybody with more incentive to notice and tell us that "Best" is selling a crappy product than the competing firm "Nature Made". Open competition in which people can make any claims they can reasonably defend as true could solve a lot of the problem.