I hate to seem on the "thief"s side, but the fact is that until you have proof that it's a stolen bicycle, ie. reported as stolen with serial number, original owners claim that it's his, or equal, it isn't a stolen bike, and calling the police won't do any good.
Your suspicion that it must be stolen because the price is too low, or the seller seems cagy, doesn't rise to reasonable suspicion or probable cause, so police - properly - will pass on this.
For all you know this is a third hand, or crashed, or whatever bike, and you were dealing with the rightful owner.
As they say, possession is 9/10ths of the law, and the owner (the guy with it in his possession) owes you or the police nothing by way of explanation or proof.
BTW- if the doorman called the police, and they came, odds are that they would have told you to go away without even speaking to the owner.
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