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They might have cross posted the bike for sale on Facebook etc. Hence the reason they took it down. Also they might be wary they can be spotted so only put the listing up for a short period of time. If you see it again arrange a meet call 999 and tell the police that you are getting your bike back with a crew and the situation is going to get violent. The police are generally useless until things are about to kick off.
999
Tell them you are recovering your stolen bike, you have a crime number, the person doesn't want to return it and the situation is getting pretty tense/you are worried there could be violence.
This is what I did last week (and every other time..).