So once a bike has been stolen (hopefully not yours), what happens to it? Well, it all depends on the level of sophistication and professionalism of the thief or thieves. The most amateur ones, which might be homeless or drug addicts looking for a few bucks for their next meal or hit, will sell bikes for a small faction of what they are worth. They'll sell it on the street, or exchange it directly for what they need, or fence it at a pawn shop or maybe a flea market or whatever.
More professional thieves want to get better prices, and they have more bikes to sell, so they can't use the same strategy. They target more expensive bikes and will often try to resell them online to get a better price. They usually will go sell bikes where there's a bigger market for them, so they might sell a bike from San Fancisco in Los Angeles.