Asking for what trouble? You weren't at the crime scene. Do you even know who was killed? Do you have a motive? Are you supposed to be responsible for the actions of another adult? Would the crime have been prevented had you not sold the gun?

Can you name one case, ever, where someone got into trouble for selling a to someone who later used it to kill someone? I think someone got into trouble after Columbine, but I think that minors were involved.