Looking back at MST3k's Invention Exchange, I see a few that would actually sell or sell with minor modifications.
Fridge Udders - Basically it is a cow patterned fridge that has milk dispensing udders. Make it a fridge that dispenses milk and it might sell.
Collapsible Trash Can - Simple brilliance. Simple and practical.
Alien Teething Nook - Come on nerd parents would but one
Rat Pack Chess set - There are other themed chess sets why not this?
Gutter Bumber Shoot - Umbrella with gutter system. Make the gutter the same material as the umbrella and it might work.