I always look at CQ as being museum quality. HEP or other professional restorer making it NIB or better grade. Honestly if it's been played a bit it loses such grade. 1 knick, 1 scratch, some dust build up on a sleeve from play anything and it's gone. A collector wouldn't play his or her game, it would sit there as a sculpture of perfection.
Imagine a motorcycle collector, they have a 1905 harley.... do they ride it? No. Do museum quality cars get driven? No. I use both of those as examples because they are both mechanical and could serve their primary purpose..... but they don't, not to a collector.
The spectrum for CQ for some is probably a little broader than I've detailed but in general it's probably top quality for all. Its over used because maybe .01% of games are of such quality but a significantly larger percentage is listed as such. The HUO designation is a much broader spectrum and not particularly useful. When someone says CQ, I usually expect the nicer side of HUO (game is fairly minty visually and is just solid, sturdy, and strong feeling is most facets).
Its tough to decide overall what everything means to you but when selling a game, in the end it's best to understate your game, be reasonable yet firm on your price and when the right buyer arrives they walk away feeling they got everything that they paid for and more.