An application of Occam's Razor would help you out a bit. This Flycrapper asshole chose an anonymous marketplace with anonymous payments to crowdfund drugs to be sent to Krebs for the express reason of being able to deny involvement as, even with a damning evidence trail such as screencaps of the forum in which he organised it, it would be next to impossible to prove he made the transaction.
Then you escalate the potential level of the attack to a level far higher than an anonymous attempt to land Krebs in trouble. Flycrapper took all steps possible to distance himself from the act (eg, trying to get others to phone in the tip) but not enough to keep it a secret from forums. Generally speaking if you're intending on sending lethal poisons to a person you don't write about it anywhere. I just don't buy that a guy who is trying to frame someone for drug possession would be happy to escalate it to murder/attempted murder and write about it in an online forum.
If Krebs wanted absolute certainty then he could have always bought a reagent test kit, which are available from most head shops and of course online.