New is better than used?
Beyond that, it all depends on how mechanically inclined you are, and what you want to do with the bike.
Do you have a specific price range, or particular version/model of ZX6 you are most interested in?
RJ is absolutely correct. Some riders hoon around on their bike, stunting, and as much as it pains me, racing. Race bikes are usually meticulously maintained, but if it gets wadded and has had a crap repair job, then most likely the frame is tweeked. If it is gonna be turned out for track days/racing then that's no major concern. But if it's intended for street use, then it might have a salvage title. Harder/higher to insure, and little resale value.
Now, for the bad news: Mr/Ms Qualify on Saturday/Bin on Sunday might have carried out the repairs at home, thus you'd have to do a bit of sleuthing, like interviewing Tech Inspectors and other Club Officers to validate this.
And if they repaired a wrecked bike and sold it privately it would be very tough to gleen the true story unless you know a thing or ten about bike repair. Which I'm gathering you don't, because you asked the Q.
A lot of racers/stunt riders would normally part a wrecked bike out themself, because there's more money to be had selling parts àla cartè via eBay/Craigslist.
Now, I don't know your budget, but the benefits of new are manifold & mostly obvious. The thing if you are not overly anal retentive, is look for 'carry-over' bikes these could possibly be 2013-'16. Usually the older the better the buy - 30th Anniversary S.E. & KRT S.E. might be a twinge more. If you are a wee bit picky, then ask them (dealer) to do a regional search for your 'must have' colour theme.