First party prototypes tend to show up much less frequently than ones from other developers. Internal security and regulations and well as desirability of many titles are probably the main reasons.
As of posting the first two have broken $300 and the TMNT remains under $100. According to the seller these came from a long time retired Nintendo employee’s large collection and he will be obtaining more games.
It should be interesting to see if any of these have differences from the retail versions. Depending on who wins them, we’ll hopefully find out.
Also, according to several proto experts on a couple of forums (most prominently NA), all signs point to these as being legit.
Also, on a non Nintendo note there’s a cool Sonic The Hedgehog Sega Genesis Mobile Display ending today.