The buy and selling of bitcoins is not illegal( well, it might be in florida. Search localbitcoin florida and you will see what I mean) ,however, the use of bitcoins to purchase illicit drugs is illegal. If the seller of bitcoins places the bitcoins in your wallet, a proprietor of illicit drugs, then that seller has unknowingly became an accomplice in the purchase, first hand not second hand like your analogy with medlabsupplies. If he sold me the bitcoins and I put them in my wallet then moved them to yours, that's different. But if I hand someone cash to put bitcoins for illegal drugs to be sent to me, they have become an accessory to a crime. It'd be the same if I had someone drop of a WU/MG order, just the same as those picking up the WU/MG orders were an accessory to a crime.
I now understand your meaning and I agree. For even logistical purposes, I would use LocalBitcoins escrow and have the Bitcoins deposited into your LBC wallet. This way, you can confirm yourself if the seller sent you the LBC, instead of waiting for me. Furthermore, by tumbling your Bitcoins, your activity in the blockchain will be very difficult to track. Does this help with reassurance? I am happy to discuss any other concerns you have.