You seem to have a good grip on your choices, though. Wait for a second-hand bike to show up, pick up something new for about twice your original budget, or order something online that you haven't been able to test ride or check for sizing. Each one has its benefits, and each one has its drawbacks. Only you can decide how the plusses and minuses weigh out.
I'm in Vancouver - shopped around a bit for my new bike in July, including south of the border and online to see what sorts of deals shook out. I was also just getting back into cycling as a way to lose weight, and knew close to nothing about bikes, bike components, or any of that when I started shopping. (Full disclosure: I still know little, but am learning fast, thanks in large part to this board).
Given my lack of knowledge, I ruled out Craigslist or used bikes; didn't want to end up with a lemon that I'd spend a fortune repairing or that wouldn't ride well. Also had little way of comparing to new bikes for pricing. Ended up checking out LBSes around town, and test rode at a few. Ended up buying from one down the street from my office as they had the best service. Bike includes 2 years of free unlimited service at the shop, which is quite nice. Also included a basic bike fitting, and discount on a full pro fit if I chose to go that route. 15% off all accessories for 2 years also. The mechanics there are super friendly and are teaching me something new every time I'm in. Gradually I'm learning to take better care of my bike myself.
I didn't find it much cheaper online or in the USA, plus I wanted to test ride some bikes here. If you go that route, be sure to factor in how much free servicing and a local opinion is to you. 20% off that $820 Cyclocross bike would be $164. Tune-ups can easily be $65 at the LBS, and fittings go a long way to helping you enjoy your bike. Final thought: small LBSes may mark up service costs or be less helpful if you roll in with a bike that you didn't buy from them.