I guess the service will be called “escrowmycoins”. 
You can protect the buyer from not losing his funds, such as if you signed first the transaction, but not the seller from fraud. The buyer can still get away, he owns his money if you sign first.
2 - Buyer submit funds to platform after both parties checking all information submitted by seller.
And why is it necessary for the buyer to deposit the money on a multi-sig address? What is it gained from the procedure if you used multi-sig from the perspective of the seller? The middleman must know that the funds of the buyer are in his possession. The seller trusts the middleman for doing so. You can protect the buyer from not losing his funds, such as if you signed first the transaction, but not the seller from fraud. The buyer can still get away, he owns his money if you sign first.
There can never be a trustless escrow system, IMO.
Correct. It is defined like that anyway.Quote from: https://en.wikipedia.org/wiki/Escrow
An escrow is a contractual arrangement in which a third party (the stakeholder or escrow agent) receives and disburses money or property for the primary transacting parties