I have never bought a new cat, but i as far as used one's go i always suggest people at the VERY least have the seller completely rig the boat prior to payment (to make sure all the parts are there, and so you can see how to rig it).

I personally would probably not buy a boat (or car) (again) without a test sail to make sure it floats and works. my first cat was missing parts and i had to spend about $400 to get it up and running.