Here is how it plays out:

He buys car from you at 2800 , promptly loses key , you tell him that a key is 300 , he grumbles - goes away - mostly forgets about you.

After falling off the wagon again, he tries to sell anything he can to get a fix. You show up offering him a fraction of what he paid, he then recalls how you " screwed him over by selling him a bad car " ( his twisted drug induced thoughts not mine ).

Desperate for a fix he sells the car to you, later when he needs more cash, he decides to show up at your door step to get money / items he can sell. " I'll get that guy that ripped me off. "

There is a reason that many drugs are illegal, it warps a persons thinking. Unable to hold a job, they lead a life of crime.