Depends on the kind of marketplace you want the escrow to deal with, but I think we already have one example of such a platform though it's limited to only 5 assets (Ethereum, Bitcoin, Litecoin, Dash, Bitcoincash)

The platform was initially known was localethereum and the escrow is based on smart contracts. It's not custodial like localbitcoins. You can even export your addresses to other wallets using private keys.