Sadly, this is an all too familiar story and there isn’t a straight forward answer.
If the thief (or handler) has anything about him then the bike won’t be at the advertisers address. It’s worth the officer looking at the pics of the bike in the ad for any distinguishing features in the background (garden features/distinctive furniture etc) as we have identified the sellers true location a few times that way.
The cops can go to the address and if they get an answer at the door, they arrest the occupant and search the address. If there’s no answer and no obvious sign of life then they will need to apply for a warrant at court.
They can of course apply for one first, providing they can satisfy a magistrate that they know where the bike is at that time.
If the officers are to try to arrange to view and buy it then this isn’t something your friendly neighborhood Bobby will be allowed to do as it’s governed by a wonderful piece of legislation called RIPA. Look it up if you suffer from insomnia – it’s basically what governs all aspects of covert policing. This is highly unlikely to be given the go ahead unless the bike is of very high value or the offender is known to be a repeat offender. And in my force it is highly unlikely that the officers authorised to do this work, would be available at a weekend for this type of case.
The officer dealing with this may not even know where to start with this type of investigation. I’m not saying it’s right, it’s just not as straight forward as you may think but it shouldn’t stop you from complaining about it.
Good luck.
Edit: a bike theft team – we briefly had such a thing in my area. It was bloody successful too. That was before we lost almost 2000 officers in 4 years.