Normally person to person buyer or seller are not legit or fake. 100,000 BTC is huge amount to scam people easily. There are lots of trusted website available in market to buy and sell bitcoins but scammers are always looking opportunity to scam people. This is shame on this industry. I got scammed in Belgium to sell BTC in cash before. So be aware of this kind of people.
person to person is infact more better or more legit imo , not the ones you transact online but im referring to meet ups. you can bring friends with you just to make sure that you are safe . while im not saying online transactions are scam but they are legit too as long your buying or selling on trusted/top platforms . using trusted escrow is also a must when doing transactiond online if you arent buying/selling from an exchange . 100k btc is something that is huge enough so i dont think an exchange can have this fund or will allow this trade