Step 1: don’t buy headphones on eBay.
Step 2: there is no step 2.
I did a very un-me thing last week after losing my beloved Sony headphones: I bought a replacement set on eBay instead of paying a little more to get them from somewhere reputable such as John Lewis.
Inevitably, they turned out to be counterfeits.
It’s not always easy to tell, but there are ways to identify fakes even before you listen to them. Slightly blurred text and inconsistent spacing on the packaging is the first tell; the lack of a warranty card is the second. If you compare the cable to a genuine pair it’s thinner and patterned differently; if you look carefully at the body of the headphones you’ll see imperfections in the lacquer.
And of course, they sound shit.
Counterfeits are a huge problem on eBay, and from third party sellers on big name sites: as a rule of thumb, anything under 72% of the usual retail price is probably counterfeit.
It’s a pain, but at least counterfeit headphones won’t kill you. Other counterfeit goods might.
All kinds of electronics are widely faked, and many of those fakes are actively dangerous: battery packs and chargers in particular have been found to be significant fire risks, and you shouldn’t buy them from anywhere you don’t trust or for prices that seem too good to be true.