I agree with [MENTION=953]Jon[/MENTION]_T and [MENTION=7263]JeffN[/MENTION]vk---Bikes Direct is a great source for low-cost, high-quality bikes.
Also check local shops for 2017 models that have been sitting forever, of some 2018 models that haven't moved---you have to get really lucky to find the right bike in the right size, but you can get good deals that way.
I also agree with this:
Originally Posted by
cyccommute
My general advice is that if you have to ask, you shouldn’t go used. I buy and build a lot of used bikes frames and used components now but it took me a lot of year to be confident enough to do that. I’d suggest buying new for the first few bikes you own until you get a good knowledge of what makes a good bike.