Yes, yes, and not very fast. (different areas have different norms, but Madison is pretty bikey, so used bikes run high and tend to be in good shape. good for noobs, bad for someone looking for a "deal".)

If your commute is short (say, 3 miles or less), any bike can do the job. So if that's your situation, don't angst about it. Pick up a used bike that feels decent, and ride. A $500 bike that you ride every day is a better deal than a $100 one that you don't ride. Fit does matter, but a lot of fit is figuring out that some part of your bike's setup is hurting you... and that bikes should never *hurt*.

The more you ride, the better a deal your bike is. And the more you ride, the better you understand bike fit, the stuff *you* need for your commute and the stuff you *like* for your commute.

If you haven't done a lot of bike repair, you're best off buying a bike in person after a test ride. Figuring out that the brakes work and the gears shift and the seat adjusts is easy in person, even if you don't know much about bikes. If you buy online, you need to assemble the bike yourself (or pay someone else to do it), and that's not a good plan for a lot of people... even mechanically inclined ones.