Hi, Timo! Welcome to ECF!
As Topwater said, it seems, sadly, that the only way to be sure you're buying originals is to buy them from reputable vendors.
The best way to guarantee you're buying fakes is if the price is significantly cheaper than from someone like RTD. The vendors he listed have genuine batteries, at a fair price.
If you're vaping on CE4 heads it may not make a difference. If the battery matters, it does.