This is a little tricky but you get nice stuff for it and I recommend you save before you try it as it might take a few tries...

Go near the Rivet City entrance and one of the random traders.

Trade him all of your stuff or until he has no caps left and then shoot him once with something small.

He and the guard will attack you, so go near the cars to the east and they should follow you.

When they are close put your gun away and they should stop. If not load from when you saved before and try again.

Stand far back and get a large gun and shoot the cars til they are on fire and make sure you pay attention to the trader as when the car explodes he'll fly away.

Then find the body and steal his caps and stuff including the stuff you sold him... you can also sell the useless stuff to another trader.

Keep in Mind: when I tried it the guard lived and attacked me and I did get bad karma for finishing him off... (I also lost the body... which sucked).