How about punching a policeman, instead, and cut down on the time until you are transferred to a cell?
I don't think it's actually illegal to do that. Just a bit morally off.
They were sent to you, you have a right to sell them (so long as you're in a one-party-consent state when it comes to recording). You're up to snuff law-wise.
Definetly a shady thing to do.