I’ve had good luck on Facebook Marketplace and Craigslist.
#1 rule know the market and don’t overprice your bike.
#2 Either list a price as firm or expected low balls and deal with it.
#3 Unless you know the person personally do not take anything but cash. Not ever. No exceptions. There is literally no other safe way to buy or sell.
#4 If you can’t trust the person to meet at your house or at a public place, don’t buy or sell with that person.
#5. Bring a witness with you, and trust your instincts. If you’re not comfortable making a really high dollar transaction, and that’s valid, then buy from a Bike shop.
There are always risks. I’ve met people at police station parking lots before. Typically as a seller I just have them come to my house. For a particularly large sale usually I will meet in a public place to keep my address a secret so they don’t come back and steel it after refusing a sale, but usually I just don’t worry about it. If people are flaky or off I don’t make the sale. If I’m buying something expensive, I go with a witness and or armed to a public place. Be smart, and do what makes you comfortable.