Do some searching around the site. Lotsa good info if you're considering a zx14.
IMO, if you are a cash buyer, $6000 is too much for the bike you described. Sounds like a $5000 bike to me, and that is dependant upon what you mean by "registration needs to be taking care of". Expired tabs? That's not a big deal. But if you mean there's an issue with getting it registered, that's a deal breaker.
Other issues I have with the bike as you describe it:
aprox 2-3k miles
Well which is it?
Back tire needs replacing
At 2-3k miles? Someone did a couple burnouts on a tight engine.
There are couple minor scratches from kid knocking the bike over on its side.
Shit happens, but if you're anything like most zx14 owners, those scratches are gonna bug you. Replacing plastics and/or painting costs $$$.
It is an all black in color.
Dems da slowest. ... actually the '07 is a metallic black. If it isn't metallic, it isn't factory.
In the end, however, if you like the bike... it's a good deal.
GL with your search.
"Life should not be a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside in a cloud of smoke, thoroughly used up, totally worn out, and loudly proclaiming "Wow! What a Ride!” HST