Cops are hoping to catch people engaging in illegal sales, and who are actually dumb enough to take up the cops on this offer to use the parking lot as a safe haven? (If you think this isn't possible, look around for stories about idiots calling the cops because someone stole their stash, or the idiots with outstanding warrants who get lured to the police station by the PD running a raffle and claiming the person won an item they can pick up at the station.)
Cops have installed spiffy new facial recognition software in their surveillance system, and they want to start keeping track of the cash transactions that take place via CraigsList?
There is simply no way this is actually a good faith attempt to benefit the citizenry here. None.