I've used a variety of craigslist search and notification apps to great success. Want a 60 cm steel frame road bike for under $200? Tell it to look for one, and it will notify you one crops up.
The problem with these marketplace notification apps is that the really good CL deals are all gone in minutes to hours, before the app has a chance to find them and notify you. If you're really bargain hunting, you basically have to troll your local CL page yourself.
I'm pretty sure everyone else uses them too, because I commonly list an item on CL at a very attractive price, hear crickets for like three or four hours, then suddenly get five texts/calls in a matter of minutes.