Couldn't reverse tracing work? A fed poses as a customer, and uses a PO Box to receive goods, but also registers this in the Fedex, USPS, UPS databases, such that if this address appears anywhere on a package, it is traced to determine the origin. Build a network over time, repeat.
The sellers drop the packages off in mailing boxes, ideally far from where they actually live. This would render your idea less worthwhile.
There have been arrest of sellers and buyers, primarily due to noobish mistakes. All told, the risk for the seller is much lower than dealing with a variety of forces in the street in their local jurisdiction.
Therefore your idea becomes a war of attrition for law enforcement. Where the yield is too low to justify the cost. This is why more effort goes into shutting down the marketplaces, but even this has become a war of attrition ever since the Silk Road 1.0 bust.
The federal government operating through just one agency got 80 million in bitcoin from that bust.
The next year, the FBI required cooperation with 16 agencies across Europe in a dragnet bust that only got 1.2 million in bitcoin, with most sites being back online in a week.
The trend continues, and every enforcement action simply prompts consumers, merchants, and site providers to implement UX and security practices that they already knew they should have been using (multisig addresses instead of human escrow, use of tails/whonix, not logging in as admin from a public library, etc)
The important point is that every time law enforcement removes one marketplace, customers are forced to visit the next one that's still online, thus increasing their revenue, effectively strengthening the strongest competitors every time they make a bust. The weakest marketplace is wiped out by law enforcement - leaving behind increasingly resilient markets - and new ones appear all the time, adding to the number of markets at the same time they're growing more resilient.
It's a fight against evolution that I can't possible see anyone winning.
They could build a Physical Onion Router. Send package to a node, which opens it to find a smaller package already addressed to another location. Similar to TOR each hop will increase security at the expense of resource utilization.
It works if you have a decent volume and can't trace individual inputs to individual outputs. If you get one package and send one package, it's a little obvious.
The FBI cannot legally trace USPS mail without a warrant, but can with Fedex, UPS, et. al. Almost all US darknet shipments go through USPS for this reason.
This is where good opsec would come in. Use postal boxes in many different locations. Be wary of cameras that could get an image of you or your plate. Don't carry your smartphone.
