Pose as a stranger and arrange to buy it.
Then either try to pass the info to the police including the meeting time and place, proof of ownership etc, or turn up with as many mates as you can and take it back.
Same happened to me some time ago. Admittedly posted for sale by a good man who bought it in good faith. I bought it back for what he paid.
If I were you and it’s worth it, just buy it back, but offer as little as possible.Chances of police help is negligent
My mates stolen bike has just turned up for sale on fb market place. What’s the best plan of action from anyone that has had a successful outcome ?!