With respect to in-person purchases of bicycles at suspiciously low prices, what a lot of people in San Francisco do is purchase the bicycle and list it as found on Craig's List(!). If someone comes forward and successfully identifies the bicycle, they buy it back at the purchased price and everyone walks away happy. If no one comes forward and identifies the bicycle, it has a grinning new owner.
Services like Craig's List are, after all, good in essence so long as they are used that way...
San Francisco, CA