The safest place to sell bitcoin would be established and reputable exchanges like Coinbase, Kraken, Binance, etc. Beyond these there are P2P exchanges like Local Bitcoins and Paxful, these are not as safe as you are dealing with a person on the other side of the transaction directly, however they circumvent central authorities (the exchanges) which could have reporting requirements etc. Nonetheless dealing with reputable sellers and buyers on these P2P platforms can be considered relatively safe.