I'd certainly not exclude ebay and Amazon, at least not in Germany (and probably some other parts of Europe).
As soon as it is a "branded part" for a regulated area of craftsmanship, local dealers will only sell at full list price, which is often way beyond reasonable pricing.
However these dealers don't have a problem with selling these parts on ebay or Amazon (or from their own online store), in this case they don't hurt anyones' business within their own regulated area.
So for your stuff you want to install on your own, shopping online is the way to go, and fake products are very uncommon, at least, if buying from an obviously established german company. Of course, don't buy from dodgy chinese companies (which sometimes do have a registered company in Germany, but still dodgy products).
Just two examples: I bought my premade solar cables on Amazon, from a large local company, which did venture into solar business, but quit solar again, and sold their solar inventory pretty cheap (I think it was like 15 EUR including shipping for a 5m mains cable with plugs plus 5x 1m panel cables with plugs. Obviously good quality.) Annoyingly, it was cheaper for my 4 solar panels to be shipped across the country than buying them locally, by like 30%.