The key to eBay isn't the "idea". It's the investment of time and effort into implementation and marketing.
I will grant that there are some ideas which are, by their nature, sufficiently non-obvious but self-fulfilling that the mere act of thinking them up has value. My practical method for interfacing a RAM to an Atari 2600 bus connector (which has neither the phi2 nor r/w signals) might qualify (at least if such ability were useful outside the Atari 2600 homebrewing community). If I were to offer a short verbal explanation for the method, anyone knowledgeable in CPLD design would be able to impement it; I can relate from experience that it works beautifully. I would posit that the idea is non-obvious as evidenced by the fact that nobody else has done it even though all necessary technologies have existed for well over a decade (someine in 1984 would have had to have used custom silicon rather than a CPLD, but some cartridges of that area use more complicated custom silicon than what would be required here, so it would have been feasible).
In most cases, however, an idea is worth little until all the gaps necessary to implement it are filled in. Even in the case of my game cartridge memory design, I doubt anyone in 1983 would have been interested in paying me for my idea without proof that it would actually work.