Yeah I've noticed unofficial dev stuff floating around as well. I think it was Andrew that showed me basically a mini DVD on Ebay that someone claimed was a gamecube prototype. It was either the worst scammer ever, or devs just hacked the gamecube and burned their own discs.You used to see them quite regularly back around 2010 - that was when the GBA was discontinued and most the developers started dumping their gear. One strange thing about the GBA was that even a lot of licensed developers were using pirate flash carts for testing - the original official Nintendo cart was huge and although they later made a reduced size "S" flash cart it was quite limited in capacity.
The pirate carts were smaller (same form factor as a regular cart), flashed faster, supported larger ROM images and were a lot cheaper.