The wider that "secret" information is spread, the less likely it remains a secret.
Lose their license? Only if they are lucky. They can be jailed for cooperating with the feds, and they know it. This is never documented, it's all done verbally. At any time, BATFE can "conveniently" discover the dealer was making known illegal sales, and burn him.
"The BATFE told me it was okay" would be disallowed in court as hearsay, and ridiculed. This entire process is designed to imprison both the dealer and the fed's snitch at the government's convenience.