Everyone lives in very separate locations, so each 'sting' would cost thousands of dollars per college kid to arrest in coordinated efforts, and some would fail because the address recipient wasn't the drug buyer.
edit:
Nobody has explained yet how the police could get good feedback, which is necessary to make lots of sales. Are they going to hack the site? lol
This is trivial. Bust a drug dealer, offer him or her prosecution immunity in return for setting up sting op. Done deal.
So meanwhile the drug dealer is still selling drugs to maintain good feedback??
Bro, listen. I already explained this. Take 12 agents. Have 1 agents pretend to sell many times to the other 11 and have the 11 leave glowing feedback. It does not take much to get reputation on SR. Have you visited it? http://ianxz6zefk72ulzz.onion/ via Tor.
And then two other people try to buy drugs and it drops below 100%. Sting fails.
No, that's not how SR works. If you try to buy something from a dealer, he doesn't have to oblige and future buyers don't require that certain other buyers have already left positive feedback. It just takes some good reputation to lure in a buyer, and it doesn't take much.