Remember that vendor selling a potato?
It's been enough time now that it's definitely okay to share the real story. People in the comments actually came pretty close.
u/zman3000 figured this was a way to test that your purchases weren't being interfered with. Well, it wasn't the deliveries that were being examined, it was the market itself.
This was an academic exercise. The vendor was a fellow student of mine who needed to set up an account and actually put something on offer in order to search for flaws in the security of the market.
He wanted to use something that wasn't illegal and probably wouldn't be bought. I suggested "baby shoes, for sale, never worn," but he thought that was too creepy.
Anyway if I remember correctly he found that the market itself was (as far as he could find) secure, and that any exposure was most likely from irresponsible behavior on the part of the user or vendor. He did NOT actually deliver anything to anyone and check for interference, however.
He's working for the FBI now, so... better implement that potato drop checking technique.
Already been done. Some guy used to send sand....you know....like from a motherfuckin beach.