I'm looking to buy or build a new bike. If I build one, it'll probably be out of the best cheap frameset and wheelset I can find and some parts I already have (tiagra shifters, 105 derailleurs, ultegra 9 speed casette.)

Or I could buy a used bike from craigslist. What do you guys think is the best route?

And how much is a good price to pay for this bike I saw on craigslist?