You're confusing the SNES with the GameCube. The eBay ad says that the consoles have been modified to output component. Something the SNES does not do natively. Also, the only proprietary thing about the cables used on the SNES is the design of the connector itself. Anyone can easily make 3rd party cables for it and the N64. And they do.
The GameCube on the other hand, at least the 001 models, has a digital output for the component cables. The cable itself has a chip in the connector that converts from digital to analog and decrypts the digital signal, as TDIRunner pointed out. Nobody has made a 3rd party cable because it wouldn't make sense financially.