The actual cost of electronic components is pretty low, when bought in large bulk. There's still some labor and capital cost involved in the actual manufacture of the stuff, but the smaller/simpler it is, the lower those costs.
A large cost in building products comes from research, development and testing, as well as marketing, as well as customer support, as well as distribution and logistics.
If a company can clone some existing product, they cut out research, development, and probably testing costs. If they sell on eBay or Amazon only, there's no marketing cost. If they provide no customer support, and distribute using regular mail, then those costs are pretty low as well.
It sounds like the capitalist market, done right! Unfortunately, it's not sustainable, for a few reasons.
First, someone needs to do the research, development, and testing of the original products.
Second, when sellers are operating on razor thin margins, if anything goes wrong (and it will,) it's the customer who's impacted, not the seller.
If a seller doesn't have to spend money building up a brand and relation with a customer, that seller can also just burn an identity every six months and pop up under a new name when the feedback gets too bad.
All in all, you don't always get as muach as what you pay for, but you pretty much never will get more than what you pay for.