Users selling pets have been updating the thread with their sells and buys, so if you scan through you might see the pets you're interested in selling or buying.
I sold a Lich Lord for 90k + 2 ectos, guy turned around and was selling it about 10 minutes later for 100k + 5 ectos... that's the way she blows.
The min for the greens and golds in the guide are a great starting point, haggle from there.