I think your intentions are great but unfortunately that idea couldn't possibly work. LE & governmental agencies are not required to comply with any privately-imposed restrictions on what they can & cannot do & read if they're trying to investigate possible illegal activity. LE is allowed to lie if its necessary in the course of a particular investigation or task. For instance, undercover police trying to nab drug-dealers or prostitutes are legally entitled to say "No" when asked by the dealer or prostitute if they are cops. I imagine that same entitlement would apply to your suggestion...if LE has any inkling that there are illegal drug/med sources listed here (and they do), they certainly don't have to abide by the TOS you suggested.

I am definitely NOT knocking your suggestion - its great that you care for DB so much that you're coming up with ideas to protect the site (and please do keep them coming!).
I think the best bet is for members to be very careful about what they post, particularly about high-profile meds like Rohypnol, GHB, Ket, Oxy, etc. & any possible sources. The PM function is great for passing along info that should remain discreet as well as being a tool for chat.

JMO & again, thanks for coming up with suggestions. Keep them coming!