Say Fred was to make a number of purchases - or just a single purchase - on DNM on Week 1. (Purchase intended for personal use; transaction used Tor, PGP, tumbler, home network - no Tails). By Week 3, the purchases & packaging are gone (e.g, they have been ingested, flushed, scattered in wind, hidden so no one could find, etc). At this point, is Fred home free, as far as LE is concerned? Assuming no future transactions.
And JUST FOR THE SAKE OF ARGUMENT assume that "hidden" really DOES mean that no one could find it OK. (And this actually IS a hypothetical scenario; hence "Fred")
LE is perfectly capable of costing you a fortune and ruining your life even with no evidence or a chance of conviction.
If they want to...
As a buyer in such case, you are probably home free. As a vendor, not until the longest SOL runs out. LE is perfectly fine with using the most sketchy violation of a law to bust people they can't directly. Tax evasion, money laundering, fake ID, lying to investigators. Whatever trap they can get you in.