Not everything on eBay is fake, there are people selling proper stuff on there.
However people selling Wylex RCBOs for £10 are either making a huge loss on every sale, or there is some other reason (i.e. stolen/fake/already been used somewhere else/etc.).
Same as anything else - if people are offering items way below normal prices, it is very likely there is some problem with the seller, the items, or both.
