Why do some members stiff me?

i bought a door on another site a few years ago from a jackass member that is on here and he totally stiffed me. i paid with a check and really didnt know any better at the time. funny when i saw him here after he got banned from that site and vanished for a few years i sent him a pm just to let him know i didnt forget about him and of coarse he denied it then had amnesia, till i gave him every possible detail i remembered about him including part of his address. then he stopped responding... again. he wanted emails and proof from over 5 years ago, which i dont have, i know i have the check somewhere but even if i show it to him im sure hell try to tell me i recieved the part. its a lost cause, but i believe in karma, and i just know not to buy from the guy again.