Parts pirates are bounty hunters of human flesh, illegally catching AWOLs and selling them on the black market. Parts pirates started surfacing when the Cap-17 law was passed. Parts pirates are notorious for setting up desperate yet very dangerous traps.
*Although it is implied that parts pirates were created by the Cap-17 law, this is countered by the novella Unstrung, which takes place before the law was passed. Thus, a more likely explanation is that parts pirates were already around before the Cap-17 law, but they simply gained more opportunity due to the sudden shortage of parts caused by the Cap-17 law.