This happened to me recently, purchased a bike recently in good faith did all my usual checks (frame number,searched forums etc) nothing came up.
Sold on eBay, police come to door with a description of a stolen bike that matches this one, the officer had a list of specifics about the bike ,ie the brand of The aftermarket pedals that had been fitted but no frame number or in fact anything that could not have been picked up from the pictures on my add.
The officer was pretty good really, I had kept the advert from where I had purchased the bike which he was happy with,no mention of handling stolen goods as could prove that the bike was purchased in good faith.
I was told to keep hold of bike while the investigation continues at the end of all this it becomes a civil matter between myself and original owner as to who owns the bike, of course I am more than happy to return the bike to original owner, as getting your bike stolen is the worst feeling as I found out for the second time last week when my shed was broken into again.