Where are stolen bikes sold?
I think that there are two kinds of bicycle thieves:


Opportunistic 
Professional


An opportunistic bike thief is most likely and opportunistic petty criminal, getting their hands on whatever they can. This kind of thief is just looking for pennies on the value of the stolen items, and will likely try to sell them at a pawn shop or flea market. I suggest checking local pawn shops and the like with your bike receipt and serial number in hand. Do this ASAP 

If your bike was expensive and your lock was cut, you may be dealing with a more professional thief. These folks will go through greater pains not to get caught by parting out the bike, or selling it in a different location. In this case, keep your eye on Craigslist in both your area and areas around you, on websites like letgo, or on local Facebook buy/sell groups.

Good luck!