Myth: A fitter can fit you on a close-to-the-right-size bike.
Reality: You have to start with the right size frame.

Whenever Greg Combs, a master fitter at Velo Smart bike shop in North Carolina, gets a fit request that starts with, “I got this bike on Craigslist...”, his warning radar goes off. “There’s a misconception that people can buy something that’s close to the correct size, and the fitter can fix it,” he says. 

But bike fit isn’t just about getting the right saddle height and the perfect-length stem to hit your reach number. It’s about putting the rider on the bike in a way that’s good for comfort, power transfer, and handling. With the wrong size bike, one (or more) of those has to give way because the physical geometry of the frame won’t allow optimal fit. Know what you will need bike size-wise before you pull the trigger on that bike you found online. It might mean paying a visit to your fitter before you start shopping.
When I finally went to a fitting, I found out why my knees and back hurt while and after riding. I don't think I'll ever buy a bike without a proper fitting