That demo cart is back up for auction. This time the seller makes it clear though there are no games on it. It was just something to show off what Sega Channel would look like :(

http://cgi.ebay.com/Sega-Channel-Demo-Cartridge-Sega-Genesis-RARE-PROTO_W0QQitemZ370287880624QQcmdZViewItemQQptZVideo_Games_Games