So this issue implies that there is a startup idea? I think there is a technical approach to solve the issue, by providing a system to let the escrow service provider to inspect and confirm the transferring of the source code, something like using a remote desktop (something like a temporary VPS) to where both the seller and the buyer have access, and during the delivering of the source code, the seller upload the code, compile it and run the result, and all the process is recorded. That way, if the buyer don't mark as 'received' in a given period, the platform staff can check and confirm the delivery and close the transaction.