Edit: No seriously, look it up. You can't legally sell weed in DC the but you can possess. So the shops sell you $70 socks and then gift you devil's lettuce.
There was that King of the Hill episode with the failing bookstore converted to a gun store, didn't have a license to actually sell guns so they started saying something like "Free gun with $600 book purchase" or something along those lines which I assume would be illegal real world, but worked in show