The Prices Paid listing shows most of the N3 sales in the $8000 - $8800 range, with one outlier at $12000 three years ago.
Jun 2010 . . $ 8800 . . Los Angeles, CA
Apr 2010 . . $ 8212 . . Ohio
Nov 2015 . . $ 8600 . . NYC metro
Apr 2016 . . $ 8800 . . Frederick, MD
Aug 2016 . . $12000 . . NYC
Sep 2017 . . $ 8000 . . Fountain Valley, CA
If the N3 in question is nearly new then $6000 is a good deal.
But if it's nearly ten years old ... not so much.
Are you sure you aren't listing prices for N2s? N3s remained around $10000-13000 up to the release of the N3X.
Personally, $7k+ isn't what I'd consider a good price for a used N2. They've been available for ~$8k new for years now, and the N1X currently costs the same new and has dramatically updated technology (new samples and resonance engine, binaural headphone samples, USB audio interface, etc.). When you consider that shops typically throw in delivery, you may be spending more on this used one than a brand new one would cost.
If I was in the market for an N2, I'd buy a new one with full factory warranty/support, or look in the $5k range for used (but AGs are very rare in the used market).