I dont think a sell has anything to do with market cap because there is both a seller and a buyer. You are selling for 100,000 but someone else is buying for 100,000. I think the market cap is based on the value of each Bitcoin which is 100,000 in your example.
The case here is the amount he wished to sell which will not be recorded on the blockchain. With that, I don't think the entirety of the market cap will be affected if you are to sell your bitcoin outside of the public ledger which is the blockchian. One other thing to consider if they are selling huge amount of bitcoin and to escape the higher fees with it is to sell the entire account itself to the buyer, which is for me, not really beneficial to the growth of the market and will just exploit vulnerabilities to our identities.