I disagree to an extent on motorcycle helmets. Yes, never ever buy a used helmet. ("Open box" might be okay.) Never buy a helmet off Wish, Ali Express, or similar; buy helmets from reputable motorcycle apparel dealers, like SportBikeTrackGear, ChapMoto, or RevZilla. But beyond that, you need to look at safety ratings. Don't get anything other than full face if you actually plan on riding, and that includes modular helmets (the chin bars tend to collapse in a crash). DOT approved (in the US) is bare-bones, and a helmet that's only DOT approved is not worth getting. Snell is... Okay. It's a North America-only standard. ECE is the global standard for helmets. The UK's SHARP - Safety Helmet Assessment and Rating Programme - probably gives the overall best idea of how effective a helmet will be in real-world situations. (SHARP ratings can be frustrating at times, because they don't let you sort readily by date; that means that you can end up seeing a lot of helmets that are no longer produced, and newer helmets may not be listed at all.)
An AGV K1 is going to be as safe as an AGV Pista GP RR, and will be about $200 v. >$1400. My $150 AGV K1 is a better helmet in every respect than my Shoei Neotec, despite the Neotec being about 4x the price.
At the upper end, you're paying for fiberglass or carbon fiber shells (less weight), better aerodynamics, better airflow, and better sound isolation. You notably aren't paying for better safety.
There are new helmet safety standards that started to be rolled out about 3, 4 years ago; not many helmets meet the new standard yet, and it's not clear whether it's a serious improvement on old standards or not. IIRC the new standard is mostly affecting helmets that are used solely or primarily on the track at the moment.