If you're not familiar with escrow services they are a mutually trusted 3rd party. You provide your source code, build tools, etc. - everything needed to re-create the distributed artifacts - to the escrow service and they verify that has been done. Should you go out of business or some other event in your sales contract is triggered the escrow company will provide those materials to your customer. My recommendation would be to find a reputable escrow company you like and utilize them. You can have them on the ready when you're creating your sales contract.