http://bazaarboom567hsuxjspmwurpl7lyx23p7r2byg22vwfhv5yubvvezid.onion/faq/for-vendors/dead-drops-guide
Take a good picture with an exact mark and warn a buyer that they will need to dig a little. A buyer should have something for digging, like a knife, sushi-restaurant food sticks, or keys. Such stashes are reliable and can safely be stored for a long time, but the packaging must be airtight.