Security is the biggest concern with anything p2p. How can you assure users that they will receive what they purchase at the agreed upon price?
That's the reason I have never used p2p services and have always stuck to exchanges like bitstamp
Both exchanges I mentioned are using an escrow system to ensure you will receive the amount you paid. There is also another exchange, which has implemented its escrow on a smart contract, which means no human interference in the procedure. Also, seller must confirm he received the money in order for the escrow to release the cryptos to the buyer.. There are some mechanism in place to ensure a smooth transaction.