He he, speakers will let you know if you're going to blow them up. If you have half a ear (and I'm surprised at the number of people who do not) you'll be able to hear the speakers in distress, telling you to knock down the volume.
The two things that kill speakers are clipping distortion and mechanical over-excursion. Clipping distortion occurs when you run a low-powered amp into a set of insensitive speakers. Since you actually get very little sound output, you turn up the volume more and more, till the amplifier starts clipping. Mind you, this is not very easy to hear. This kind of distortion fries tweeters very quickly.
Mechanical over-excursion is when the applied input power increases to the point that woofer moves so much that it can no longer stay in the magnetic gap. This increases the chances of mechanical breakage of the woofer bits. Won't get more technical than that. This can also happen to tweeters in a badly designed system. I have little idea of how your pannies are constructed but as Superczar said, if you watch the volume control a bit, you should be fine.
Now to your question of value. The price of used equipment varies with condition and your own negotiating skills. Also they don't depreciate quite as much as, say, home theater stuff. They also last a great deal of time.
Is 10K a good deal? I dunno, that's why you're going to have to check it out. You should bargain the price down as much as you can, but be sure to check out the amp. I repeat, don't buy sight-unseen.
I hate to break it to you, but with those speakers, I'm not sure how much of the amp you're going to be able to judge. The NAD is meant for good speakers, and on a temp pair you might only be able to tell if the amp is broken or not.
But if they're only temporary, they'll do. Pick up a pair of Tannoys or PSBs to go with it later, your ears will thank you.