If Satoshi Nakamoto will indeed sell some of his BTC, it will create panic in overall market because he is holding millions of BTC on his wallet, It's enough to dump the market to the starting price of BTC. But I guess he will not do that because someone can track him in case he cash out his BTC to Fiat since many eyes are watching on his wallet address since the beginning.
But I do not think that Satoshi Nakamoto will sell some of his bitcoin in just one order. I think he will send his bitcoin to reputable exchanges without having KYC, and get the money to the other forms. But I think he already cashes out his some bitcoin to enjoy what he has. He can use the other services available on the dark market, so they can not track or watch on his wallet. I am sure he can sell his bitcoin without we know if he is Satoshi Nakamoto or other people.
We may never know what could happen with those bitcoins satoshi owns, it is not something we can even predict. If satoshi is real and it is just one person, and if he is alive, he could simple come down here and sell all of his bitcoins and use that money to live rich forever, but honestly you see what is going on with XRP so I doubt it would be the case, if he did that there would be a ton of people and governments who will want to question him and probably even jail him.
But what could happen would be some hacker finally managing to get into satoshi's wallet, I know it is very very difficult but there must be some people who have been working on it for years now, and if you give it enough time and juice, we could potentially see something like this happen, it might take a long time but it could at least happen with enough time, so I guess we could see those bitcoins sold on market after a successful hacking.