how to sell code/rights of incomplete app project

This can be done in two ways, sell the company which holds title to the copyright, or assign the copyright to the buyer. Both require similar documentation, that the employees assign all copyright to the company, waiver of rights or non-compete from previous primary developer, a contract indicating what assets/rights are being transferred (including trademarks or trade secrets), and any ancillary data such as potential customers lists or beta testers. Getting 3rd party validation of the market is a good step but often hard due to uncertainty as to the traction (no working product). You have to realise that code alone might not be sufficient as the knowledge is embedded inside the brains so the contract may require being available to transfer the knowledge to the new owner.