Based on the video footage the eBayer posted, there's nothing even remotely "prototype" about this cartridge.
People who don't work in the game industry just loveto throw around the word "prototype" or "beta" for any kind of pre-release game disc or cartridge that wasn't actually released 100% in its pre-release form. The reality is that a "prototype", in game industry nomenclature, is typically reserved for an extremely early build of the game, well before the game has even gone into production let alone a testing cycle. "Alpha" versions of games are content- and feature-complete, and are playable, but typically lack polished art, have performance issues, are significantly buggy, and may still undergo major revisions. "Beta" versions of games, excluding the ones that typically undergo public beta-testing due to the enormity of doing a full sweep, are typically nearly identical to the released version of the game save for crash bugs.
Based on the cartridge ID "NES-SRP-TEST-02", it's obviously a cartridge that was used in the QA (testing) department for doing sweeps of bugs, well into the final development cycle. I'd even bet dollars to donuts, based on the complete lack of anything different in the video versus the released version of the game, that it was one of the last builds of the game made before it went to manufacturing.
That being the case, is the game even remotely worth $150,000? Hardly. Maybe if there were significant differences between it and the final version of the game, maybe if it was actually different in some way, but it's not. This guy is just trying to make a buck off of collectors with more money than technical sense.