The only problem with using the word Prototype is that the Ebay storm troopers tend to pull down auctions with this term since it is against Ebay policy to sell beta versions of games. You might want to try using the terms "Development", or "Sample", or "Demo", it is a difficult balance between keeping your listing up and having it pulled. Though I recently did see an auction use the term prototype for a Mega Man NES cart and it wasn't pulled. So you might want to just put it up using prototype and see what happens. Though from what I have seen any time the word prototype is used in the listing header the auction tends to get pulled.