I'll answer the general questions to the best of my ability.
1. Usually, you get a bill of sale from the seller, they fill it out and sign it as proof that they're selling you the car, then they sign the title over to you as well. You bring the title and bill of sale down to the DMV, they'll give you your registration, and you call your insurance company and add the car to your plan
2. Subtract money for whatever is not easy and cheap to fix. A little scratch that can be buffed out? Don't hassle the seller too
much. The ****ed up trunk that needs to be replaced rather than repaired because it looks like someone dropped a ton of bricks on it? Yeah, knock a few hundred off if it's a cheap car, even more if it's newer or more expensive.
3. From owner with good paperwork is best, IMO, if you're paying cash. Usually easier to get it from a dealership if you're going to be getting a loan for it, I'd imagine, since most of them work closely with banks. Both small and large dealerships do sketchy ****, so beware either way.
4. There's no 100% price. It depends on your area, the seller, the buyer, the car in particular, etc. Selling prices on any car are going to vary a good bit.