There's a reason some folks who do electronics repair call eBay "FleeBay".
Counterfeit parts are a huge problem in electronics. Usually, buying parts through authorized distributors is safe. But there are also the "grey market", reasonably reputable disties who are not authorized distributors, and the black market, sellers of unknown reliability and honesty. The grey and black market is risky, even if the sellers are honest companies/people, because they often don't know the true provenance of what they sell.
At a previous employer I worked in Component Engineering. Usually will did testing to qualify parts for new projects or as second sources. On occasion we tested parts from a grey market source prior to our company buying the part. This usually was when there were temporary shortages.
On one occasion we got sample power MOSFETs that had correct markings, but unknown provenance. Through our tests, we were able not only to tell the MOSFETs were not genuine, but also were able to tell what the parts really were (a very good but lower performance part).
On another occasion we were sent a batch of electrolytic capacitors. The parts were such bad counterfeits that I didn't even need to open the bag they were sent in. The plastic sleeve was the wrong color, the font of the markings was wrong, and the date/source code format was wrong. They were laughably bad counterfeits!
Back on-topic, had we used those parts, we probably would have experienced poor reliability as a consequence. Counterfeit lithium ion batteries, though, could be deadly-dangerous. Poor quality batteries could cause fires.
