What is an escrow:  An escrow is a method to allow two trustless parties to make a trade giving to a trusted third party the minimal trust needed to hold the coins until the trade is completed. This tursted third party is called escrow.

There is a few p2p marketplaces that  offer a decentralized, non-custodial escrow system.

The technical details of the escrow depends on  the platform and token choosen. 

A non-custodial escrow system makes it impossible for the admins of the platform to steal from users wallets or escrow accounts. By the same design, the platform's non-custodial nature makes it immune from common hacks and thefts.

What happens if buyers doesnt pay:  A decentralized escrow account holds the crypto side of the trade. This provides a guarantee of funds to the buyer, and an abort path for the seller.

The buyer of crypto should never send money before the crypto is in escrow. Likewise, the seller should only release the escrow after they see money in their account.

If payment never arrives, either party can raise a payment dispute. This allows an arbitrator to decrypt messages, verify evidence, and get the crypto to its rightful owner.

Why is p2p decentralized exchanges better. You've read the news. Exchanges have lost the password to hundreds of millions of dollars worth of Bitcoin. They have accidentally deleted at least $15 million worth of ETH. They collect troves of sensitive passport scans only for them to end up on the dark web and sold to identity thieves.

Centralized exchanges lost lots of billion of clients money in the last years. 

Crypto has earned itself a poor reputation, but it's not because of the technology. It's because Wall Street bankers have crept into the space and created a mess.

When you trade on a non-custodial crypto marketplace, you are in control—not custodians, not bankers.