The GameCube is "officially collectible," says GameStop, in a notice that, come April 2, it will no longer accept trade-ins on any GameCube consoles, games or accessories. It's at the bottom of the retailer's trade-in page.
That doesn't mean your local store will stop selling its GameCube consoles, games or accessories, of course. GameStop will likely continue to sell off that stock until it all runs out. But if you have any of that stuff laying around and you want store credit for it, you better dump it all now for the 99 cents they probably give you. For all of it.
There are many stages of a console's death. This is the last one. The GameCube celebrated its 10 year anniversary last fall, and was discontinued in 2007. It lasted longer as a trade-in commodity than the original Xbox, probably because of the Wii's backward compatibility. But as that console is nearing the finish line, it inevitably meant the end for its predecessor, too.
GameStop Says Goodbye to the GameCube [Nintendo World Report]