Why would he go to the trouble of filing/grinding off the serial numbers (if he's the one who did it), then leave the receipt lying around? Then again, if the guy was a whackjob, who the heck knows. Sigh.
Maybe the person who sold it to him wanted to have it be untraceable.
I noticed that he wrote “you caused me to do this”. How could the parents not notice that he needed serious help? Sure he was a young adult, but crikey, what is family for but to tell you what you need to hear?