Hi Robert357, you can, in fact, sell the firearm you build. The ATF makes a distinction between a manufacturer and a builder. The wording of the relevant law only states one cannot build a firearm with the intent of selling it. Building a homemade firearm and deciding to sell it later is perfectly legal. Midway's guide is centered on assembling a rifle itself, whereas this guide focuses entirely on completing fabrication of the receiver blank. If you'd like more detailed information on building an AR-15 - and all the other necessary parts and steps - I recommend starting off here,
How to Build an AR-15: The Complete Parts Guide.