As you can imagine, you can't buy a salvage auto, strap it together with duct tape and drive off down the freeway. Decaying, decrepit and damaged cars are a danger not only to their drivers, but also to everyone else on the road. For this reason, you'll have to jump through the hoop of a rebuilt auto inspection before you can license your salvage auto.
Depending on your state, inspection must either be by any licensed body shop, a body shop specifically licensed to inspect rebuilt autos or a police officer. Most states also require a specific addition to the registration paperwork, detailing the damage and repairs. And some states have an entirely separate office for licensing rebuilt autos. Again, check your state's specific requirements: Here are links to the 50 state DMVs.