Seattle Bike Blog
had an article today about recovering a stolen bike (and it was quite an adventure!). The thing that caught my eye was a web-based tool called ifttt.com
..."If This Then That." While not exclusively bike related, it was instrumental in that bike recovery. It could also help if you're looking to buy a particular brand or model.
Read the website for more details, but basically it lets you set up "if/then" rules for its webcrawler. So, for example, you could tell it to do something like "If [Schwinn Varsity on Craigslist] then [send me a message]." It scans lots more than just Craigslist, like all the social media sites and more.
I haven't tried it, not vouching for it, just looks like a handy 'net tool to know about.