Cycletrader and cragislist are great resources.
However, I caution you against anything below a 600 and here is why. Speaking specifically about sport bikes or crotch rockets as you say, the lower the CC's on the bike the harder it has to work for you. If you do freeway travel you are likely to be hovering at about 65 to 70 depending upon your level of comfort. a 500cc bike will work 25% harder than a 600 cc bike. The 25% may be off, but it's really the point not the math.
On a cruiser or upright bike it is similar in the since that the bike will weigh more and the lower the cc the harder it works and the heavier it is going to have to work even harder.
Starting, since it has been 5 years I'd settle for nothing less than 600cc. But...in a couple of years you may grow out of the bike so used is not only your best bet with price but with growth factor.
There is a yamaha 600 I don't recall the actual name but I can look in my garage later and tell you that is a very good bike for a 1st timer or someone who hasn't ridden in a while.
License:
The process of getting your license is to walk yourself down the the dmv and take the written test. If you pass, you get a provisional license that lets you ride during daylight hours only and not on the freeways...highways according to the CHP are ok (walked into the station and asked). You spend a few months riding like that if you can afford to ride only in daylight and then you go to a community college or go online and spend a few hundred dollars to take a rider's safety course, take a test, and get your license that way. It's the easiest way to do it and you get some experience in bike safety. If you drive in California then you know that the majority of the riders have nothing to fear about the bike except for the moron in 10 seater SUV beside you putting on makeup, threatening to smack her kid if they don't shut up while talking on their cell phone to their friend who has been waiting for them to go to the mall for the last 20 minutes.
The place I am talking about that isn't a community college is U-Ride Motorcycle training.
If you buy a used bike always bring someone who knows the ins and outs of bikes. You want to make sure that engine is good and some people will lie and tell you the bike hasn't been laid down when in fact it has.
Case in point:
Yamaha 750, 2005 with 1200 miles on craigslist for $2800. This bike would retail for well over $9k used. It had a new seat, new gas tank, new bar grips, new break leavers, new tail lights, new exhaust. They were all changed from chrome to black because he liked the look. Turns out he failed to change the pegs and it was apparent the bike was laid down in an accident. When asked to test ride he agreed and brought out his helmet that matched the pretty bike. The helmet had a large scratch on the side...it looked a lot like asphalt hitting the helmet...when asked about it he confessed. Had there been no request for a ride this may not have been known. You generally want to stay away from newer bikes that are being sold for far less and have "new extras" put on.
Good luck and stay safe.
