From what I've read/gathered they screen people with interesting items and have them come in when they have the store empty and ready for filming. I know that they screen via internet write ins, and I would assume walk ins.
This is called staging or setting up a scene Sam
I think the negotiation is real.
No, it's not. Predetermined by the producers.
Rick may have foreknowledge of the stuff, but he seems to pretty much stick with the value the experts say, so I dont see much point to him knowing alot about items beforehand.