For one thing, this assumes all buyers and sellers are legitimate in the system.

The best way to map a drug network is to become a real part of it.

Just like in the real world, it wouldn't be hard to pretend to be a seller to catch a few buyers - not hard, but also not worth doing - whereas by pretending to be a buyer you aren't going to find out the address of a seller.

It might be worth doing, a lot of buyers are probably small time sellers too and may well have links with other parts of the drugs network.

I can certainly imagine there is value in controlling something like SR as part of a larger investigation.