This is a matter of faith and luck. But, you can ask for pictures and read the feedback information. You will get better used gear from Audiogon than much other used markets in my experience.
Although your intended pieces are not that expensive new, you will have a much better price if buying used instead. If the difference is not that big though, you should consider your local diller since he will give you full guarantee. In my experience, NAD has a good quality and they last a long time. If you end buying the Paradigns used, be sure to buy them from a near seller if possible to avoid damages in shipping.
Good luck and enjoy your new system,
Regards,