The flashback scenes show "Robert Johnson" playing a Martin guitar (probably 000 or OM) model. This is entirely fictional as the few photos of Johnson that exist show him playing a Gibson or possibly its cheaper cousin, a Kalamazoo. Strange how movies continually get these details wrong when there are a whole host of books and experts able to advise?
When he is walking towards the final showdown duel, you can hear him walking in hard soled shoes but when he bends down to pick up the guitar cord, it is plain to see he is wearing soft tennis shoes.