Well, man, I see that many people here suggested you to use Binance, but this exchange is centralized, so it’s not exactly what you’ve been asking for. True decentralized systems are a bit complicated and they have low volume, so think about traditional
P2P networks like LocalBitcoins. Strictly speaking, they are decentralized too but they are more user-friendly. The thing is that such sites place sellers’ offers which can be viewed by buyers. It’s important to pay attention to merchants and read their terms carefully to make a reliable deal. Also, It’s better to look at number of positive reviews, so you could choose the best trader.