Well thats funny b/c last time I was in that same store (according to your pic's receipt) I had a conversation about rare 7800 games with another fellow customer, he was near the counter/register, & I was by the door/Playstation games so we were basically talking the length of the store, so anyone in the place HAD to hear us- including the owner.
I mentioned I used to have some of the rarer 7800 titles- Ninja Golf, Commando, Alien Brigade, but since sold them years back. & Im pretty sure the other guy started rattling of what these titles are currently worth.
So the owner almost certainly knew what he had there...usually that place is too expensive for my tastes, but thats a damn good find.
Honestly I'd be really surprised if the store owner didn't know what Ninja Golf was worth, since I stop in there and shoot the breeze with him once or twice a month about Atari games and he's always been very knowledgeable about pretty much everything relating to Atari. I actually bought my Light Sixer model 2600 that I use almost every day from him and most of the 2600 games I own are from that store as well.
So if I had to venture a guess I'd say he probably did know what Ninja Golf was worth and just gave me a ridiculously good deal on it because I've bought so much Atari stuff from him in the past, and I'm guessing he was feeling sorry for me since I had just had surgery and was in pretty rough shape.